Improvement: Classpath is Incomplete warning should give more context

Issue #193 points out that this message can be less than helpful. The wiki points out that this occurs when opening individual java files. But the cases associated with this message suggest there are various other conditions - missing .project files, misconfigured .classpath, among others. I have an environment where I open multiple project directories in a single vscode window. I don't even know which project to start looking at when this message appears.

It would be helpful if the message could display "Classpath is incomplete" in (File or directory reference)

Environment

• Operating System: OSX
• JDK version: jdk 1.8
• Visual Studio Code version: 1.35.1
• Java extension version: 0.46.0

Steps To Reproduce

1. With (say) 3 java project directories in a vscode window, delete .project in one.
2. Java: Clean the language server environment

"Classpath is Incomplete" warning is displayed

Current Result

Expected Result

Additional Informations

Start with issue #193 and trace the existing "Classpath is incomplete" warning in the Java extension. Reproduce it with multiple Java project directories in one VS Code window after deleting a .project file and cleaning the language server environment. Done means the warning identifies the relevant file or directory.