loader: provenance in LoadErrors from _get_loader

Angelos Evripiotisrequested to merge
aevri/junction_load_provenance into master

Give better error messages to the user by including provenance. Pass 'provenance' through to places where we raise exceptions in _get_loader.

For example, here you can see the provenance information now included (the bit before the ':'):

callHello.bst [line 8 column 4]: Could not find the project.conf
file in the project referred to by junction element
'hello-junction.bst'.

Helps #960 (closed)

Edited by Angelos Evripiotis

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