BMesh: remove redundant mesh-backups from EDBM_op_* API

Using BMesh operators through the edit-mesh API created a full copy
of the mesh so it was possible to restore the mesh in case
one of the operators raised an error.

Remove support for automatic backup/restore from the EDBM_op_* API's
as it adds significant overhead and was rarely used.

Operators that need this can use the BMBackup API to backup & restore
the mesh in case of failure.

Add warning levels to BMO_error_raise so operators can report problems
without it being interpreted as a request to cancel the operation.

For high-poly meshes creating and freeing a full copy is an expensive
operation, removing this gives a speedup of ~1.77x for most operators
except for "connect_verts" / "connect_vert_pair"
which still uses this functionality.

source/blender/python/bmesh/bmesh_py_ops_call.c

@@ -44,10 +44,10 @@ BLI_STATIC_ASSERT(sizeof(PyC_FlagSet) == sizeof(BMO_FlagSet), "size mismatch");
 
 static int bpy_bm_op_as_py_error(BMesh *bm)
 {
-  if (BMO_error_occurred(bm)) {
+  if (BMO_error_occurred_at_level(bm, BMO_ERROR_FATAL)) {
     /* NOTE: we could have multiple errors. */
     const char *errmsg;
-    if (BMO_error_get(bm, &errmsg, NULL)) {
+    if (BMO_error_get(bm, &errmsg, NULL, NULL)) {
       PyErr_Format(PyExc_RuntimeError, "bmesh operator: %.200s", errmsg);
       BMO_error_clear(bm);
       return -1;