Catch exceptions from StringMakers inside Detail::stringify

This stops tests failing falsely if the assertion passed, but the
stringification itself failed as the assertion was sent to the reporter.

I don't think that stringification should be fallible, but the
overhead in compilation ended up being small enough (<0.5% on `SelfTest`)
that it might be worth implementing, in case there is more users
with weird `StringMaker`s than just MongoDB.

Closes #2980

src/catch2/catch_tostring.cpp

@@ -8,6 +8,7 @@
 
 #include <catch2/catch_tostring.hpp>
 #include <catch2/interfaces/catch_interfaces_config.hpp>
+#include <catch2/interfaces/catch_interfaces_registry_hub.hpp>
 #include <catch2/internal/catch_context.hpp>
 #include <catch2/internal/catch_polyfills.hpp>
 
@@ -113,6 +114,15 @@ namespace Detail {
              rss << std::setw(2) << static_cast<unsigned>(bytes[i]);
        return rss.str();
     }
+
+    std::string makeExceptionHappenedString() {
+        return "{ stringification failed with an exception: \"" +
+               translateActiveException() + "\" }";
+
+    }
+
+
+
 } // end Detail namespace