NSRegularExpression: Improved Error and Exception handling to match macOS behaviour

[hmelder]

When creating a NSRegularExpression object on macOS, the following error message is created.

NSRegularExpression *exp;
exp = [NSRegularExpression regularExpressionWithPattern:@"(abc" options:0 error:&error];
if (!exp) {
  NSLog(@"%@", error);
}

Output macOS 14.0

2023-11-11 19:43:08.886 objc-boilerplate[7287:328207] Error Domain=NSCocoaErrorDomain Code=2048 "The value “(abc” is invalid." UserInfo={NSInvalidValue=(abc}

On GNUstep we did not populate the error value.

Additionally, we did not check nil values:

NSRegularExpression *exp;
exp = [NSRegularExpression regularExpressionWithPattern:nil options:0 error:&error];

Output macOS 14.0

*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** -[NSRegularExpression initWithPattern:options:error:]: nil argument'
*** First throw call stack:
(
        0   CoreFoundation                      0x000000018f4a88c0 __exceptionPreprocess + 176
        1   libobjc.A.dylib                     0x000000018efa1eb4 objc_exception_throw + 60
        2   Foundation                          0x0000000190535ce0 -[NSRegularExpression initWithPattern:options:error:] + 664
        3   Foundation                          0x0000000190535a28 +[NSRegularExpression regularExpressionWithPattern:options:error:] + 52
        4   objc-boilerplate                    0x0000000102113df0 main + 928
        5   dyld                                0x000000018efdd058 start + 2224

This PR aligns the GNUstep implementation with Apple's Foundation framework, and supplies test cases for testing correct behaviour.

[rfm]

OK. It would be better if the changes matched GNUstep coding style (but I can easily fix that).

[hmelder]

OK. It would be better if the changes matched GNUstep coding style (but I can easily fix that).

The whole file uses two space indentation which is not up to the GNUstep coding style.