Evalml: Update automl search "raise_errors" flag to default to true

In #252 , @angela97lin fixed a bug which went undiscovered because our unit test coverage of automl fit was configured to not raise errors (specifically, the parameter raise_errors currently defaults to false). Her PR updated the unit tests to set raise_errors to true in each call to automl fit in the tests.

I wanted to create a ticket to discuss this a bit further. I'm confused about the raise_errors parameter. Why does it exist? Why does it only apply to automl fit? If it is a necessary parameter, is there a better design, and in particular one which would discourage bugs like this? I'm concerned that if we rely on the team to remember to set raise_errors to true in the unit tests, we could run into similar issues again.

Ideas: I know @kmax12 mentioned using an environment variable for this. If this parameter is necessary but my unit testing concerns is valid, this could be a good solution. We could also update all the tests to use a test fixture which somehow makes sure raise_errors is set appropriately, either via env var, other global config, or by wrapping the automl fit method.