More descriptive error message on failure to unzip .ibc

To give more guidance on errors like
https://github.com/edwinb/SDL-idris/issues/2

The DecompressError can be one of three things:
http://hackage.haskell.org/package/zlib-0.5.4.1/docs/Codec-Compression-Zlib-Internal.html#t:DecompressError
There's not a show instance for it, though.
When I try to unzip "/etc/passwd" with this, the text of that error
message returned is "incorrect header check".
Merge pull request #786 from LeifW/report_unzip_errors

More descriptive error message on failure to unzip .ibc