altosui: Display error message when parsing pyro channel values fails

altosui: Display error message when parsing pyro channel values fails

Build an exception handling chain to get numeric parse errors
propagated all the way back to the original 'save' command and up into
a dialog window, including the pyro channel, field and value that were
in error.

Signed-off-by: Keith Packard <keithp@keithp.com>