WIP: Use context_for_bsterror to provide provenance on error

Description

Provide provenance information in the error if project.conf is missing from a junction, like so:

callHello.bst [line 8 column 4]: Could not find the project.conf file in the project referred to by junction element 'hello-junction.bst'. Was expecting it at path 'project' in the junction's source.

Changes proposed in this merge request:

• Add BstError.add_context() that accepts stringifiable things to add to exception error messages.
• Add a custom BstError.__str__ method, so context is prepended like so: 'context: original message'.
• Introduce a new context_for_bsterror context manager, which does the try, except, and add_context() for you.
• Fixup some instances of using "{}".format(e) instead of str(e).
• Add a usage of context_for_bsterror.
• ... TODO: more usages of context_for_bsterror.

This merge request, when approved, will close:

Helps #960 (closed)

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