[Telemetry] Fail explicitly if --profile-dir argument doesn't exist

Telemetry currently fails if the value of --profile-dir is not a directory however the failure is just a byproduct of normal processing.

This CL makes the failure explicit and adds a more verbose error message.

BUG=None

Review URL: https://codereview.chromium.org/52093009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232106 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 232106