Error with caching of Essence' when other bugs occur

[jwatts-maybe]

Probably the simplest way to recreate this issue:

1. Take a known good Essence model, run Conjure to solve it.
2. Run Conjure on it again, this time with the -qi flag, which (per a separate bug report) trips Conjure's bug message mid-compile.
3. Attempt to run Conjure "normally" again on the model.

This returns the output:

Using cached models.
No solutions found.
Multiple eprime models found in the output directory, not copying solutions.

In actuality, there are no Essence' files in conjure-output at this stage.

More generally, this occurs whenever a bug prevents Conjure from completing compilation to Essence' between "good" runs. For example, I have also demonstrated this behaviour by taking a file with a line in it that causes Conjure to crash (specifically my toMSet example from a separate bug report) and commenting/uncommenting the bad line between runs.

The behaviour is also the same if you just delete the .eprime file, so I assume this is just a way for #495 to manifest from Conjure deleting the file but not replacing it.