Reword the github diagnostic summary

[saecki]

I think the previous wording contributed to repeated discussions about warnings that should indeed be fixed.
Still open to tweak this new summary though :)

[elegaanz]

To me an error means the package won't work at all, and a warning means it will work but doesn't follow best practices. Warnings exist to encourage package authors to follow these practices, but if they want to submit a low quality package despite the warnings, I think they should be allowed to do so. If you feel like we should be more strict about warnings and force people to fix them unless they really are false positives, we can discuss it a bit more, I'm not strongly opposed to that.