Improve FppMissingSupportFiles error message

[thomas-bc]

Originating Project/Creator
Affected Component
Affected Architectures(s)
Related Issue(s)	nasa/fprime#1888
Has Unit Tests (y/n)
Builds Without Errors (y/n)
Unit Tests Pass (y/n)
Documentation Included (y/n)

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Change Description

Fixes nasa/fprime#1888.

I wasn't sure about the wording, this is the best that I could come up with in a concise manner. Let me know if you can think of better.

[LeStarch]

It seems like the commands should work independently of current working directory. Do they not do this?

[thomas-bc]

The command is looking for the fpp-input-list under the build cache in the same context as where it is ran from. So if the current directory does not define FPP files, fpp-input-list is not generated by CMake in that context, and the command errors.
https://github.com/fprime-community/fprime-tools/blob/0c668dc8145ef5b806f6f892723ca5694f00d58a/src/fprime/fpp/common.py#L78-L81