avocado.core.exceptions: Return TestNotFoundError

This test exception was lost, even though we use it in
the definition of the avocado.test.MissingTest class.
Signed-off-by: NLucas Meneghel Rodrigues <lmr@redhat.com>

avocado/core/exceptions.py

@@ -86,6 +86,14 @@ class NotATestError(TestBaseException):
     status = "NOT_A_TEST"
 
 
+class TestNotFoundError(TestBaseException):
+
+    """
+    Indicates that the test was not found in the test directory.
+    """
+    status = "ERROR"
+
+
 class TestTimeoutError(TestBaseException):
 
     """