Running dep init in $GOPATH/src returns misleading error

[dcheney-atlassian]

Repro

% env GOPATH=$(mktemp -d) bash -c 'mkdir $GOPATH/src && cd $GOPATH/src && dep init'
determineProjectRoot: /tmp/tmp.P8wqCD27Aj/src not in any $GOPATH

The error is misleading, /tmp/tmp.P8wqCD27Aj/src is $GOPATH/src

[sdboyer]

Yeah - this is the same basic use case as raised in #313. Until we get to actually supporting it properly (#417), it deserves its own special failure message.

[nijaru]

I'll take a look over this!