Small fixes

[Framba-Luca]

This PR fixes 2 problems:

• a bug in the UsertypeFluentsRemover when used in a compiler's pipeline
• the error message given by the factory when an engine is not selected due to the missing optimality gurantees

[codecov-commenter]

Codecov Report

Attention: 13 lines in your changes are missing coverage. Please review.

Comparison is base (da5bc97) 84.95% compared to head (5a560f0) 84.92%.

Files	Patch %	Lines
unified_planning/engines/factory.py	25.00%	12 Missing :warning:
...ning/engines/compilers/usertype_fluents_remover.py	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #552 +/- ## ========================================== - Coverage 84.95% 84.92% -0.03% ========================================== Files 200 200 Lines 26428 26448 +20 ========================================== + Hits 22452 22462 +10 - Misses 3976 3986 +10 ```

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