Correct mispelling in cache_file.py

This corrects a misspelling in error handling code that would cause an
irrelevant error to be thrown when unable to access an existing cache
file. This misspelling issue would only appear after a fatal error had
occurred, so there is no change in behavior.

Signed-off-by: Raymond Wagner <rwagner@mythtv.org>
Merge pull request #1 from wagnerrp/master

Correct mispelling in cache_file.py