goto-instrument: add early output file validation

[tautschnig]

Validate output file presence before expensive analysis operations to prevent wasted computation when command is malformed.

Fixes: #2593

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

❌ Patch coverage is 82.35294% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.00%. Comparing base (1505c36) to head (a8315e7).

Files with missing lines	Patch %	Lines
.../goto-instrument/goto_instrument_parse_options.cpp	82.35%	9 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8784   +/-   ##
========================================
  Coverage    80.00%   80.00%           
========================================
  Files         1700     1700           
  Lines       188254   188305   +51     
  Branches        73       73           
========================================
+ Hits        150615   150657   +42     
- Misses       37639    37648    +9     

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