charm attach should check charm url and give error if version is not specified

[Jay R. Wren]

attach.go

e.g.
charm attach cs:~bigdata-dev/insightedge InsightEdgeDiff=InsightEdge-1.0.0-juju-diff.tar.gz

uploads the tarball and then returns an error that says version must be specified.

expected:
check that version is in charm url before uploading and give nice error message if it is not.