Improve validation for invalid files in schemas dir
Previous validation of schema files was not very specific if
there was problems with files. Now there is more direct message if
something wrong with yang files.
Removed logic that rename not yang files into .yang in schema dir.
We are storing yang models only in .yang files. Therefore all other
files should be rejected.
JIRA: NETCONF-707
Change-Id: Iad4b91fd534eef8ea134a9463de3dab618fe969e
Signed-off-by: OleksandrZharov <Oleksandr.Zharov@pantheon.tech>