Allow the user to interact with the svn password prompt
if needed.  Verify the svn server is reachable before
proceeding.  Clean extra UI issues up.  Readability
fixes.

BUG=none
TEST=prompted for a svn password if ~/.subversion/auth
is empty, bad svn server causes early timeout
Review URL: http://codereview.chromium.org/267059

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@29552 0039d316-1c4b-4281-b951-d872f2087c98