Allow composability of unhandled exception filters (#3033)

The change is very simple. If a handler previously existed, Catch2 will invoke it after printing out its output. I've also updated the comment to better reflect that it's returning EXCEPTION_CONTINUE_SEARCH even in scenarios where the exception is one that the library cares about.

src/catch2/internal/catch_fatal_condition_handler.cpp

@@ -86,23 +86,27 @@ namespace Catch {
         { EXCEPTION_INT_DIVIDE_BY_ZERO, "Divide by zero error" },
     };
 
+    // Since we do not support multiple instantiations, we put these
+    // into global variables and rely on cleaning them up in outlined
+    // constructors/destructors
+    static LPTOP_LEVEL_EXCEPTION_FILTER previousTopLevelExceptionFilter = nullptr;
+
+
     static LONG CALLBACK topLevelExceptionFilter(PEXCEPTION_POINTERS ExceptionInfo) {
         for (auto const& def : signalDefs) {
             if (ExceptionInfo->ExceptionRecord->ExceptionCode == def.id) {
                 reportFatal(def.name);
             }
         }
-        // If its not an exception we care about, pass it along.
+        // If a filter was previously registered, invoke it
+        if (previousTopLevelExceptionFilter) {
+            return previousTopLevelExceptionFilter(ExceptionInfo);
+        }
+        // Otherwise, pass along all exceptions.
         // This stops us from eating debugger breaks etc.
         return EXCEPTION_CONTINUE_SEARCH;
     }
 
-    // Since we do not support multiple instantiations, we put these
-    // into global variables and rely on cleaning them up in outlined
-    // constructors/destructors
-    static LPTOP_LEVEL_EXCEPTION_FILTER previousTopLevelExceptionFilter = nullptr;
-
-
     // For MSVC, we reserve part of the stack memory for handling
     // memory overflow structured exception.
     FatalConditionHandler::FatalConditionHandler() {