More user-friendly reporting on include errors

Description

This MR adds more information to include-related errors that are reported to the user.

Changes proposed in this merge request:

• Mention the including file when include is not found.
• Mention the including file when include is a directory.
• Be more accurate about the line no. when reporting recursive includes.

For example, previously including a dir would result in an error like this:

mydir is a directory. bst command expects a .bst file.

Note that the file containing the include was not mentioned.

Now we get an error like this instead:

element.bst [line 12 column 0]: Include block references a
directory instead of a file: 'mydir'.

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