Improve Error message of Safeloader

[richtja]

Is your feature request related to a problem? Please describe.
Right now, when the safeloader is not able to load tests from the test file, it doesn't provide any information about why tests were not found. This might be problematic when test writer make mistake like badly named test methods like this:

#! /usr/bin/python3

from avocado import Test

class MyTest(Test):
    def firsttest(self):
        pass

    def secondtest(self):
        assert False

with this example, the user doesn't have any information for debugging this problem:

avocado -V list test.py
Type Test Tag(s)

Resolver             Reference          Info
avocado-instrumented /tmp/wrong_test.py
golang               /tmp/wrong_test.py go binary not found
python-unittest      /tmp/wrong_test.py
robot                /tmp/wrong_test.py File "/tmp/wrong_test.py" does not end with ".robot"
exec-test            /tmp/wrong_test.py File "/tmp/wrong_test.py" does not exist or is not executable
tap                  /tmp/wrong_test.py File "/tmp/wrong_test.py" does not exist or is not executable

TEST TYPES SUMMARY
==================

Describe the solution you'd like
Let's provide more information form safeloader to debug this problem, something like this.

avocado -V list test.py
Type Test Tag(s)

Resolver             Reference          Info
avocado-instrumented /tmp/wrong_test.py no methods called `test_*`
golang               /tmp/wrong_test.py go binary not found
python-unittest      /tmp/wrong_test.py
robot                /tmp/wrong_test.py File "/tmp/wrong_test.py" does not end with ".robot"
exec-test            /tmp/wrong_test.py File "/tmp/wrong_test.py" does not exist or is not executable
tap                  /tmp/wrong_test.py File "/tmp/wrong_test.py" does not exist or is not executable

TEST TYPES SUMMARY
====