Inform user that generate test action can't be called in some situations

[EgorkaKulikov]

Description

Sometimes we can't generate tests for users code. The generation process will fail for some reasons, sometimes they do not depend on us. We should avoid generation in such situations and inform user about the reasons.

It should highly likely be done similarly to the notifying that test generation during indexing is impossible.

Expected behavior

1. If user tries to generate tests for an abstract class, we should avoid generation
   see Test generation for an abstract class fails #873 for more details

2. If users's code is located in a package which name starts from "java", we should avoid generation
   see https://stackoverflow.com/questions/5490555/prohibited-package-name-java

May be there are some another situations...

Potential alternatives

Fail test generation or do not react on users request at all - seems not very good.