Improve the error handling of Run Java button #1092
Description
This is a follow-up with the issue microsoft/vscode-java-pack#832 (comment).
I can simply reproduce it by dragging in a .java file without opening any folder.
Run Java: silent failure
Debug Java: silent failure
F5: silent failure with a debugging toolbar.
We should probably optimize debugger extension for such case. Beginners might get stuck on this due to silent failure.