Defining malformed scopes for dependencies produce confusing error messages

[esuomi]

When defining scopes for dependencies in malformed way the produced error messages are not very helpful, at least for newbies. Examples:

[some/dep "1.2.3" :provided]
;=> "Provided artifact is missing a version: [some/dep nil "1.2.3" :provided]"
[some/dep "1.2.3" :scope :provided]
;=> "clojure.lang.Keyword cannot be cast to java.lang.String"

The correct form for defining the scope of course is

[some/dep "1.2.3" :scope "provided"]

The error message should probably either describe the full form of the dependency vector or potentially try to autodetect what the user is trying to do and suggest a fix based on that, whichever makes more sense.

[technomancy]

Sure; I'd take a patch to improve that error message.