dapp install with no arguments has ugly error message

[asymmetric]

Instead of showing git's failure at cloning an empty string, it could print a usage blob (also considering there's 3 different syntaxes for adding libraries)

[kmbarry1]

Taking this as a warm-up bug; should have something soon^TM.

[kmbarry1]

Also, what's the third syntax? I know one can do the following, which I assume are the first two:
dapp install ds-foo
dapp install https://github.com/dapphub/ds-foo

The third seems to be some sort of relative path thing, but I don't see how it's intended to work.

[rainbreak]

@kmbarry1 the third syntax is dapp install github_org_name/ds-foo, e.g. dapp install makerdao/dss.