PY-21339: Use AppConfig for INSTALLED_APPS for Django 1.9+

* AppConfig added
 * #getTopLevelClasses() marked @NotNull since the only implementation is @NotNull
 * QualifiedNameResolverImpl verbosity improved

python/psi-api/src/com/jetbrains/python/psi/PyFile.java

@@ -27,6 +27,7 @@ import java.util.List;
 public interface PyFile extends PyElement, PsiFile, PyDocStringOwner, ScopeOwner {
   List<PyStatement> getStatements();
 
+  @NotNull
   List<PyClass> getTopLevelClasses();
 
   @NotNull

python/src/com/jetbrains/python/psi/impl/PyFileImpl.java

@@ -367,6 +367,7 @@ public class PyFileImpl extends PsiFileBase implements PyFile, PyExpression {
   }
 
   @Override
+  @NotNull
   public List<PyClass> getTopLevelClasses() {
     return PyPsiUtils.collectStubChildren(this, getStub(), PyElementTypes.CLASS_DECLARATION);
   }

python/src/com/jetbrains/python/psi/resolve/QualifiedNameResolverImpl.java

@@ -393,7 +393,7 @@ public class QualifiedNameResolverImpl implements RootVisitor, QualifiedNameReso
       RootVisitorHost.visitSdkRoots(myContext.getSdk(), this);
     }
     else {
-      throw new IllegalStateException();
+      throw new IllegalStateException("Context is empty. Provide some context: module, file, sdk, etc");
     }
   }