rosdistro cache loading is not robust to a missing cache file

[Tully Foote]

Generating the hydro cache is blocked on this.

0 upgraded, 0 newly installed, 0 to remove and 106 not upgraded.
+ pwd
+ rosdistro_generate_cache hydro --cache /home/rosbuild/hudson/workspace/_hydro-rosdistro-cache
Exception in thread Thread-2:
Traceback (most recent call last):
  File "/usr/lib/python2.6/threading.py", line 532, in __bootstrap_inner
    self.run()
  File "/usr/lib/python2.6/threading.py", line 484, in run
    self.__target(*self.__args, **self.__kwargs)
  File "/usr/lib/pymodules/python2.6/rosdistro/rosdistro.py", line 57, in _construct_rosdistro_dependencies
    self.depends_file = RosDependencies(name, cache_location)
  File "/usr/lib/pymodules/python2.6/rosdistro/rosdistro.py", line 294, in __init__
    deps = self._read_server_cache()
  File "/usr/lib/pymodules/python2.6/rosdistro/rosdistro.py", line 334, in _read_server_cache
    tar = tarfile.open(tar_file[0], 'r')
  File "/usr/lib/python2.6/tarfile.py", line 1658, in open
    raise ReadError("file could not be opened successfully")
ReadError: file could not be opened successfully

Caching ivcon