debug edid foo - help and error could be better

[CarlFK]

the help doesn't show the full 'syntax' of the command, and the parser? doesn't respect the short form that other commands use.

"foo port has no EDID capabilities" should be "foo is not an output port" or "no such port"

  git describe: v0.0.4-165-g619dcf9
H2U 00:16:57>help
...
  debug edid                     - dump monitor EDID

H2U 00:16:57>debug edid
 port has no EDID capabilities

H2U 00:17:45>debug edid o0
o0 port has no EDID capabilities

H2U 00:17:58>debug edid output0
(works)

H2U 00:33:10>x c p 1
Connecting pattern to output1

H2U 00:33:12>o1 on
Enabling output1