pushed exception setting into the relevant function, removed redundant exceptions

The following functions set exceptions automatically:
PyImport_ImportModule, PyObject_GetAttrString, PyArg_ParseTuple. All
the exception setting functions in c_overviewer should act the same
way and set their own exceptions before returning.

Notably, PyDict_GetItemString does *not* set exceptions, so it is
handled specially in render_mode_find_interface.

overviewer_core/src/iterate.c

@@ -45,47 +45,47 @@ PyObject *init_chunk_render(PyObject *self, PyObject *args) {
     textures = PyImport_ImportModule("overviewer_core.textures");
     /* ensure none of these pointers are NULL */    
     if ((!textures)) {
-        return PyErr_Format(PyExc_ImportError, "Failed to import overviewer_core.textures");
+        return NULL;
     }
     
     chunk_mod = PyImport_ImportModule("overviewer_core.chunk");
     /* ensure none of these pointers are NULL */    
     if ((!chunk_mod)) {
-        return PyErr_Format(PyExc_ImportError, "Failed to import overviewer_core.chunk");
+        return NULL;
     }
     
     blockmap = PyObject_GetAttrString(textures, "blockmap");
     if (!blockmap)
-        return PyErr_Format(PyExc_ImportError, "Failed to get textures.blockmap");
+        return NULL;
     
     tmp = PyObject_GetAttrString(textures, "max_blockid");
     if (!tmp)
-        return PyErr_Format(PyExc_ImportError, "Failed to get textures.max_blockid");
+        return NULL;
     max_blockid = PyInt_AsLong(tmp);
     Py_DECREF(tmp);
 
     tmp = PyObject_GetAttrString(textures, "max_data");
     if (!tmp)
-        return PyErr_Format(PyExc_ImportError, "Failed to get textures.max_blockid");
+        return NULL;
     max_data = PyInt_AsLong(tmp);
     Py_DECREF(tmp);
     
     /* assemble the property table */
     known_blocks = PyObject_GetAttrString(textures, "known_blocks");
     if (!known_blocks)
-        return PyErr_Format(PyExc_ImportError, "Failed to get textures.known_blocks");
+        return NULL;
     transparent_blocks = PyObject_GetAttrString(textures, "transparent_blocks");
     if (!transparent_blocks)
-        return PyErr_Format(PyExc_ImportError, "Failed to get textures.transparent_blocks");
+        return NULL;
     solid_blocks = PyObject_GetAttrString(textures, "solid_blocks");
     if (!solid_blocks)
-        return PyErr_Format(PyExc_ImportError, "Failed to get textures.solid_blocks");
+        return NULL;
     fluid_blocks = PyObject_GetAttrString(textures, "fluid_blocks");
     if (!fluid_blocks)
-        return PyErr_Format(PyExc_ImportError, "Failed to get textures.fluid_blocks");
+        return NULL;
     nospawn_blocks = PyObject_GetAttrString(textures, "nospawn_blocks");
     if (!nospawn_blocks)
-        return PyErr_Format(PyExc_ImportError, "Failed to get textures.nospawn_blocks");
+        return NULL;
     
     block_properties = calloc(max_blockid, sizeof(unsigned char));
     for (i = 0; i < max_blockid; i++) {
@@ -362,7 +362,7 @@ chunk_render(PyObject *self, PyObject *args) {
     PyObject *t = NULL;
     
     if (!PyArg_ParseTuple(args, "OOiiO",  &state.self, &state.img, &xoff, &yoff, &state.blockdata_expanded))
-        return PyErr_Format(PyExc_ValueError, "Failed to ParseTuple");
+        return NULL;
     
     /* fill in important modules */
     state.textures = textures;

overviewer_core/src/rendermodes.c

@@ -106,13 +106,13 @@ render_mode_find_interface(const char *mode) {
     /* check for custom modes */
     custom = PyDict_GetItemString(custom_render_modes, mode);
     if (custom == NULL)
-        return NULL;
+        return PyErr_Format(PyExc_RuntimeError, "Failed to find rendermode interface (custom not found)");
     custom = PyDict_GetItemString(custom, "parent");
     if (custom == NULL)
-        return NULL;
+        return PyErr_Format(PyExc_RuntimeError, "Failed to find rendermode interface (parent not found)");
     custom_parent = PyString_AsString(custom);
     if (custom_parent == NULL)
-        return NULL;
+        return NULL; // PyString_AsString sets an exception
     
     return render_mode_find_interface(custom_parent);
 }
@@ -124,11 +124,11 @@ RenderMode *render_mode_create(const char *mode, RenderState *state) {
     
     iface = render_mode_find_interface(mode);
     if (iface == NULL)
-        return PyErr_Format(PyExc_RuntimeError, "Failed to find rendermode interface");
+        return NULL;
     
     options = render_mode_create_options(mode);
     if (options == NULL)
-        return PyErr_Format(PyExc_RuntimeError, "Failed to create rendermode options");
+        return NULL;
     
     ret = calloc(1, sizeof(RenderMode));
     if (ret == NULL) {
@@ -150,7 +150,7 @@ RenderMode *render_mode_create(const char *mode, RenderState *state) {
         Py_DECREF(options);
         free(ret->mode);
         free(ret);
-        return PyErr_Format(PyExc_RuntimeError, "Failed to start rendermode interface");
+        return NULL;
     }
     
     Py_DECREF(options);