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

mayan/apps/ocr/api.py

@@ -73,6 +73,9 @@ def do_document_ocr(queue_document):
                 document_page.content = ocr_cleanup(ocr_text)
                 document_page.page_label = _(u'Text from OCR')
                 document_page.save()
+            except Exception as e:
+                logger.debug('missing ocr backend: %s' % ocr_backend)
+                logger.debug('I/O error({0}): {1}'.format(e.errno, e.strerror))
             finally:
                 fs_cleanup(pre_ocr_filepath_w_ext)
                 fs_cleanup(unpaper_input)