Add fallback exception message

[WarLikeLaux]

Q	A
Is bugfix?	❌
New feature?	✔️
Breaks BC?	❌
Fixed issues

What does this PR do?

Adds a public ErrorHandler::$fallbackExceptionMessage property to customize the error message shown to end users when YII_DEBUG is false and an exception occurs during error handling.

Previously, changing this message required overriding the entire handleFallbackExceptionMessage() method. Now it can be set via config - as a string or a callable:

'errorHandler' => [
    'fallbackExceptionMessage' => 'Service temporarily unavailable.',
],

'errorHandler' => [
    'fallbackExceptionMessage' => function ($exception, $previousException) {
        return 'Error: ' . $exception->getMessage();
    },
],

Alternative approach to #20681, which used a protected method. A public property is more consistent with existing ErrorHandler options like $discardExistingOutput and $silentExitOnException. Callable support adds flexibility without subclassing.

Includes 3 unit tests for the default value, string config and callable config. handleFallbackExceptionMessage() itself calls exit(1), so full integration testing is not practical, but the property path is covered.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.13%. Comparing base (37b6e7b) to head (f0badce).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
framework/base/ErrorHandler.php	0.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #20780      +/-   ##
============================================
- Coverage     80.13%   80.13%   -0.01%     
- Complexity    11533    11534       +1     
============================================
  Files           374      374              
  Lines         30206    30208       +2     
============================================
  Hits          24207    24207              
- Misses         5999     6001       +2     

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