Output a useful error message when trying to load a directory

Description

Outputs a more useful error message when a command tries to load a directory instead of a .bst file

We now check to see if that directory is actually a .bst file.

Add new error message and exception

Changes proposed in this merge request:

• Added new exception.
• Catch the exception when opening a directory instead of a file.
• Return an error message to the user which is clearer than the current one.
• Added a test

Closes #446 (closed)

Replaces !520 (closed)