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

tests/SelfTest/IntrospectiveTests/ToString.tests.cpp

@@ -118,3 +118,24 @@ TEST_CASE( "#2944 - Stringifying dates before 1970 should not crash", "[.approva
             Equals( "gmtime from provided timepoint has failed. This "
                     "happens e.g. with pre-1970 dates using Microsoft libc" ) );
 }
+
+namespace {
+    struct ThrowsOnStringification {
+        friend bool operator==( ThrowsOnStringification,
+                                ThrowsOnStringification ) {
+            return true;
+        }
+    };
+}
+
+template <>
+struct Catch::StringMaker<ThrowsOnStringification> {
+    static std::string convert(ThrowsOnStringification) {
+        throw std::runtime_error( "Invalid" );
+    }
+};
+
+TEST_CASE( "Stringifying bla bla bla" ) {
+    ThrowsOnStringification tos;
+    CHECK( tos == tos );
+}