Add Custom Exception Classes to BrowserLibrary for User Code to Catch

[TWijesundera]

Currently, browser library throws an AssertionError for every keyword that fails due to timeout or assertion issues. IMO this is too broad and BrowserLibrary should raise custom exceptions for user code to catch so we can be certain that BrowserLibrary is raising an error and not other parts of the testing suite.

This practice is common with other python libraries: https://requests.readthedocs.io/en/latest/_modules/requests/exceptions/

[aaltat]

We need to discuss this with the team. I understand your needs but I am not sure how we can do this with Assertion Engine

@allcontributors please add @TWijesundera for ideas

[allcontributors]

@aaltat

I've put up a pull request to add @TWijesundera! 🎉

[TWijesundera]

I think you don't need to worry about assertion engine. You could probably transform the AssertionError raised by assertion engine into some other exception class