dont swallow [ERROR] from stderr, if glide list exits non-zero

[lalyos]

when glide list exits with non-zero, the user gets only the following output

$ glide-vc
exit status 1

which doesn't really helps.

with the suggested changes the user would see the causing error:

$ glide-vc
[ERROR] Error scanning github.com/bmatcuk/doublestar: cannot find package "." in:
[ERROR]         vendor/github.com/bmatcuk/doublestar
[ERROR] Error listing dependencies: Error resolving imports
exit status 1