Output a more clear configuration error (#421)

* Output a more clear configuration error

* Performance related improvements in config read handling

* Check python 3.6 with travis

* Revert the .travis.yml py36 inclusion

* Added tests for input error handling

* Thanks isort, isorted it out

* Make exit() function a bit more aesthetic

* Single quote -> double quote docstrings

* Whitespace removed

pytest.ini

@@ -1,5 +1,7 @@
 [pytest]
-testpaths = ./test-data
+testpaths =
+    ./test-plugin
+    ./test-data
 addopts =
     --tb=native
     -s