fix #3553

[MarcoScaletta]

Related Issue

This pull request addresses #3553 .

Intended Change

If a key environment file (for example, updateRules.key or javaRules.key) contains a syntactic error, a meaningful error message is now shown in the GUI.

Type of pull request

• Bug fix (non-breaking change which fixes an issue)

Ensuring quality

• I have tested the feature as follows:
  I tried to load updateRules.key and javaRules.key with syntactic errors obtaining proper error messages.

The contributions within this pull request are licensed under GPLv2 (only) for inclusion in KeY.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 38.39%. Comparing base (46ef350) to head (284bb1e).
Report is 13 commits behind head on main.

Files with missing lines	Patch %	Lines
...rc/main/java/de/uka/ilkd/key/proof/io/KeYFile.java	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #3560      +/-   ##
============================================
+ Coverage     38.33%   38.39%   +0.06%     
+ Complexity    17257    17238      -19     
============================================
  Files          2111     2098      -13     
  Lines        127630   127324     -306     
  Branches      21461    21442      -19     
============================================
- Hits          48923    48886      -37     
+ Misses        72695    72427     -268     
+ Partials       6012     6011       -1     

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