Make prefect linting care about lint levels

[danpetry]

i.e., it shouldn't fail on ERROR, WARNING, and INFO. Definitely on ERROR, and maybe on WARNING too.

It may be helpful also to define more clearly what type of lints go in which of these categories.

[marcoesters]

I think INFO should never result in a failure. WARNING is fine, or we could leave it up to the discretion of the reviewers to determine if that warning is worth blocking the PR.

[danpetry]

this makes sense to me: ERROR is a hard fail which doesn't require any other interpretation, and WARNING requires some interpretation to determine how serious it is.
Something like this: https://gcc.gnu.org/onlinedocs/gcc-5.1.0/gcc/Warnings-and-Errors.html

[marcoesters]

This should help with the implementation: #120

[schuylermartin45]

Tracking: https://anaconda.atlassian.net/browse/PAT-115