Fix cascading diagnostics when using invalid crypto options with PublicSign

For instance, see PublicSignWithRelativeKeyPath in CompilationAPITests.vb.

There are two errors:

BC2014: the value 'test.snk' is invalid for option 'CryptoKeyFile'
+BC37254: Public sign was specified and requires a public key, but no public key was specified

In this case, only the first error is really necessary. This is true for almost any set of incompatible crypto options with public sign, including using CryptoKeyContainer or any set of mutually incompatible options.

See https://github.com/dotnet/roslyn/pull/9130 for the full set of diagnostics produced by incompatible options.