Hotfix for "Error: undefined" in test output

[jay-tux]

Some functions in src/framework/Testee.ts catch a string as if it were an Error:

        .catch((e: Error) => {
            scenarioResult.error = e;
        });

My current fix is to have these catch the error as Error|string and then do a type-check to ensure that scenarioResult.error is always an Error|undefined:

        .catch((e: Error|string) => {
            if(typeof e === 'string') scenarioResult.error = new Error(e);
            else scenarioResult.error = e;
        });

[tolauwae]

Good enough for a hotfix.