Use a constant for common CTAs in ErrorDetails

[charlespierce]

In updating the errors to all include nicer error messages, several of them have turned out to use the same CTA (by far the most common is a CTA about permissions for file system errors). These CTAs should be put into a constant and just referenced in each error message, so that we don't have a bunch of duplication in the strings.

This should be done after all of the .unknown() work has been reviewed and merged, so that the CTAs can be updated all at once.