Atest plugin: modify error message.

Bug: 149061216
Test: 1. In atest_plugin folder: ./gradlew :runIde
      2. Open a project which not in Android directory.
      3. Launch Atest tool window, click the run button.
         It should show the error dialog "Atest can only execute when
         your project is under Android source directory.".

Change-Id: I3b041c28dba55b962b46ed87345b16c460857f50
1 file changed