Deleting a file from a release raises exceptions.

[Ohad Kammar]

When deleting a file from a release, exceptions are thrown.

To reproduce, run the following in python:
from launchpadlib.launchpad \
  import Launchpad,STAGING_SERVICE_ROOT
cachedir = #### Enter your cachedir here.
lp = Launchpad.get_token_and_login('bug reproduction', STAGING_SERVICE_ROOT, cachedir)
#### ... authorization handshake. Make sure you give the script privileges to change anything!
proj = lp.projects['name-of-project'] #### Enter the name of a project with the following properties:
#### 1. You have access privileges to delete release files.
#### 2. The project has a release with files uploaded to it.
>>> rel = proj.releases[0] #### Replace '0' by the number of the release with the file in it.
>>> f = rel.files[0]
>>> f.delete()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "build/bdist.linux-i686/egg/launchpadlib/resource.py", line 341, in __call__
  File "build/bdist.linux-i686/egg/launchpadlib/resource.py", line 452, in lp_refresh
  File "build/bdist.linux-i686/egg/launchpadlib/resource.py", line 274, in lp_refresh
  File "build/bdist.linux-i686/egg/launchpadlib/_browser.py", line 221, in get
  File "build/bdist.linux-i686/egg/launchpadlib/_browser.py", line 211, in _request
launchpadlib.errors.HTTPError: HTTP Error 404: Not Found
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[Leonard Richardson (leonardr)]

launchpadlib tries to refresh its view of the file and can't handle the fact that the file isn't there anymore.