"Invalid file path" message is confusing and misleading

[DanTup]

[Error - 10:31:25 AM] Request textDocument/codeAction failed.
  Message: Invalid file path
  Code: -32003 
/Users/danny/Desktop/dart_samples/dartcode_flutter_3825/lib/main.dart

This error message happens when the LSP server fails to get a parsed result for a filename. When the message was written, it was assumed the only reason this could occur is that the path is not valid. However, it can also happen in other cases (such as a failure to parse the file). It would be nice to understand these differences in the server and produce more specific messages to avoid everything seeming like the same problem (which often ends up with overloaded GH issues).

[DanTup]

https://dart-review.googlesource.com/c/sdk/+/232861/