Enhance the error message when importing from .MAT files with an access key (e.g. Projection Matrix importer, but other might do as well)

Original comment from Tim:
When uploading a projection matrix, in .mat format, you have to give the name of the key to access the data. When you indicate the wrong name, the error is 'projection_matrix'. Could we have something more informative like 'wrong key to access field' or something like that?