configs: Don't panic if new version constraint parser raises an error
configs: Don't panic if new version constraint parser raises an error

The new provider installer code is using a new version constraint parser
because it produces better error messages than the one we were using
before. However, it has some cases where it returns errors that the old
parser (which was entirely regex-match-based) didn't catch.

In the long run we should consistently use the new parser everywhere, but
until then we'll avoid panicking then the two disagree, by returning
diagnostic messages instead of using MustParseVersionConstraints.

For now, we only hit these error cases if the user enters something that
the old parser allows but the new parser does not.

apparentlymart/terraform,pryorda/terraform,rnaveiras/terraform,rnaveiras/terraform,rnaveiras/terraform,ewbankkit/terraform,tommynsong/terraform,ewbankkit/terraform,pdecat/terraform,hashicorp/terraform,apparentlymart/terraform,pryorda/terraform,pdecat/terraform,apparentlymart/terraform,ewbankkit/terraform,pryorda/terraform,pdecat/terraform,tommynsong/terraform,apparentlymart/terraform,hashicorp/terraform,ewbankkit/terraform,apparentlymart/terraform,hashicorp/terraform,tommynsong/terraform,pdecat/terraform