Do not fail, when ocr backend is missing (#50).

- Since tesseract (i.e. an ocr backend) is an optional requiremnt of mayan, we
  shouldn't fail, if it is not found.
- S.a https://github.com/mayan-edms/mayan-edms/issues/50
Merge pull request #52 from mbehrle/feature/unfail_ocr_backend

Do not fail, when ocr backend is missing (#50).