Log regex compilation failure in find_regex

Closes #193.

src/drf_yasg/inspectors/field.py

@@ -262,7 +262,12 @@ def find_regex(regex_field):
             regex_validator = validator
 
     # regex_validator.regex should be a compiled re object...
+    try:
         pattern = getattr(getattr(regex_validator, 'regex', None), 'pattern', None)
+    except Exception:  # pragma: no cover
+        logger.warning('failed to compile regex validator of ' + str(regex_field), exc_info=True)
+        return None
+
     if pattern:
         # attempt some basic cleanup to remove regex constructs not supported by JavaScript
         #  -- swagger uses javascript-style regexes - see https://github.com/swagger-api/swagger-editor/issues/1601