Add error details when a dependent chart's version cannot be found in a repo

[AndiDog]

What this PR does / why we need it:

This is about the error in case a chart version does not (yet) exist in the repo. Example:

dependencies:
  - name: abcd
    version: "0.18.0-does-not-exist"
    repository: "https://example.com/helmrepo"

Before:

$ helm dependency update
[...]
Error: can't get a valid version for repositories abcd. Try changing the version constraint in Chart.yaml

The error message mistakenly refers to "repositories", but the printed value is a chart name. Also, since abcd here isn't quoted, it could be mistaken as being a regular word in the error message sentence.

After:

$ helm dependency update
[...]
Error: can't get a valid version for 1 subchart(s): "abcd" (repository "https://example.com/helmrepo", version "0.18.0-does-not-exist"). Make sure a matching chart version exists in the repo, or change the version constraint in Chart.yaml

If applicable:

• this PR contains documentation
• this PR contains unit tests
• this PR has been tested for backwards compatibility

[robertsirc]

LGTM