Verify file existence before deleting it

When creating a cached session file, a temporary file is generated; this
file is then renamed to the final cache file name. Afterwards, when
trying to erase this temporary file, a warning log entry is written
because the delete operation fails (the file does not exist anymore
giving it was renamed). Fix this by verifying a file exists before
trying to delete it.

Change-Id: If7e6435ccfbd5806a3167c6f07087a1082527628