"Script error." stopped getting filtered correctly

[staticshock]

As of #1057, "Script error." errors no longer filters out, because the error type gets prefixed to the string being tested. In Chrome, this results in a testString value of ": Script error." now (the type comes in as undefined.)

Here are the relevant bits of code:

https://github.com/getsentry/raven-js/blob/70b9dd6921b74ef06c3f7bfd43fb01d09f8711e8/src/raven.js#L139-L145

https://github.com/getsentry/raven-js/blob/70b9dd6921b74ef06c3f7bfd43fb01d09f8711e8/src/raven.js#L1423-L1429

[benvinegar]

@kamilogorek I think we should only append the : if there is a valid error type, such that we never test on smoething like : <message>.