Correction to file path error messaging

[cmbenne3]

Summary

Small bug fix to improve the error messaging when aviary cannot find a file path.
Aviary now correctly displays the 'relative' path that was searched.

Related Issues

No issue number - discovered and fixed whilst debugging an Aviary aircraft model.

Details:

Aviary searches for 'path' in following order:

1. absolute
2. relative to current working directory
3. relative to aviary package
4. built in models

After each block of logic 'path' is overwritten so the next 'if' can determine whether 'path' exists.
The relative path string wasn't saved, and so the error message unhelpfully displayed the aviary_based path twice e.g:

FileNotFoundError: File not found in absolute path: concepts_library/a320neo/propulsion/aviaryPW1127G_eng_deck.csv, relative path: C:\Users\user\Git\Aviary\aviary\concepts_library\a320neo\propulsion\aviaryPW1127G_eng_deck.csv, or Aviary-based path: C:\Users\user\Git\Aviary\aviary\concepts_library\a320neo\propulsion\aviaryPW1127G_eng_deck.csv

This PR fixes the issue by saving the relative path string so that the error message can help the user identify where aviary is actually looking for the file:

FileNotFoundError: File not found in absolute path: concepts_library/a320neo/propulsion/aviaryPW1127G_eng_deck.csv, relative path: C:\Users\user\Git\ConceptsLibrary\concepts_library\a320neo\concepts_library\a320neo\propulsion\aviaryPW1127G_eng_deck.csv, or Aviary-based path: C:\Users\user\Git\Aviary\aviary\concepts_library\a320neo\propulsion\aviaryPW1127G_eng_deck.csv

Backwards incompatibilities

None

New Dependencies

None

[cmbenne3]

It seems like autopep8 has broken the f string onto multiple lines, but won't allow you to put the explicit line break character in (it gets removed by autopep8)...
I found this issue hhatto/autopep8#712 which claims to have been fixed... Any ideas greatly appreciated!