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

从 issue #193 开始，追踪 Java extension 中现有的警告 "Classpath is incomplete"。删除 .project 文件并清理 language server 环境后，在同一个 VS Code 窗口中使用多个 Java 项目目录重现该问题。完成标准是警告能够指出相关文件或目录。