hgk: don't exit if mercurial commands only print warnings

tag: allow multiple tags to be added or removed - Example: "hg tag -r 42 build-25 beta-1" will add tags build-25 and beta-1 for rev 42. - The deprecated and undocumented usage "hg tag arg1 arg2" used to emit a warning, then add tag arg1 for rev arg2 (equivalent to "hg tag -r arg2 arg1"). It will now add tags arg1 and arg2 for the current revision. - If one tag triggers an error, no tags are added/removed (all or nothing).

tests: update test-command-template for the new {isodatesec} filter