install: Return more user-friendly response on eexists

Most newcomers hit helm's 'feature' there deleted instalations are not really deleted.
Use-case
# helm install  --name gitlab-ce-1 ./stable/gitlab-ce
# helm delete gitlab-ce-1
# helm list
# helm install  --name gitlab-ce-1 ./stable/gitlab-ce
Error: a release named "gitlab-ce-1" already exists

There is a lot duplicated bugs in bugzilla which simply explains that packages
should be deleted with --pure flag. But such bugs appeared again and again
because this behavior is not obvious.

Let's help user to figure out what happens ASAP.