Give more helpful error when stratum artifacts in cache are corrupted

I had a stratum artifact in my artifact cache which for some reason was
0 bytes long. When building a system that included this stratum, `morph
build` gave me the following output

    2015-04-13 13:48:57 ERROR Traceback (most recent call last):
      File "/src/morph/morphlib/builder.py", line 539, in build_and_cache
        self.unpack_strata(fs_root)
      File "/src/morph/morphlib/builder.py", line 600, in unpack_strata
        chunks = [ArtifactCacheReference(c) for c in json.load(f)]
      File "/usr/lib/python2.7/json/__init__.py", line 290, in load
        **kw)
      File "/usr/lib/python2.7/json/__init__.py", line 338, in loads
        return _default_decoder.decode(s)
      File "/usr/lib/python2.7/json/decoder.py", line 366, in decode
        obj, end = self.raw_decode(s, idx=_w(s, 0).end())
      File "/usr/lib/python2.7/json/decoder.py", line 384, in raw_decode
        raise ValueError("No JSON object could be decoded")
    ValueError: No JSON object could be decoded

With this patch, I get a better error:

    ERROR: Corruption detected: No JSON object could be decoded while
loading
/src/build/cache/artifacts/8b4422c58ecb2a085b142fbba74b760f501f65d4b2885bf707994973230e0c58.stratum.build-essential-minimal

Change-Id: I0ad359901c5da75bd26d5a1a8108ef4e6f1d7708

0 files changed, 0 insertions, 0 deletions