Optionally guess error class when missing

[waltjones]

Fixes: https://app.clubhouse.io/rollbar/story/54135/rollbar-js-exception-class

Previously, when error.name and error.constructor.name are both missing, error.message would be used to guess the class, using a regex to look for and extract the part before the colon.

This has unintended results most of the time. In cases where the name property is missing, it is unlikely the class will be in the message string. Meanwhile, it's possible for other strings to match the regex and generate unexpected values that get assigned to be the error class. Besides being confusing and not useful, this interferes with grouping.

This PR makes the default behavior to assign "(unknown)" as the error class. This is the value already being used when no value can be guessed.

When config.guessErrorClass is set, this engages the old behavior. For any users who are intentionally adding error classes to their message strings, setting this will allow them to be extracted. Note that as before, this only happens when both error.name and error.constructor.name are missing.