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
Fix the way we try for missing binary dependencies in the installations app
Issue #50. Thanks to Ford Guo (@fordguo) for the reporting.
Don't silence OCR errors at the API high level, at this layer we don't know what really happened in the backend.
Move exception handling to the backend.
Don't silence OCR errors even if Tesseract is optional otherwise the user won't know happened.
Catch the OSError generic exception and return a friendlier "Tesseract not found" exception