* check for workspace command name arg (fix#5131)
For the 'workspace <name> output <output>' command, output_location must
be greater than zero or the attempt to get the workspace name with
join_args will segfault or abort() (depending on the flavor of
sway_assert() in use). This checks and returns an error instead.
* put workspace output error string on one line
To ease grepping as requested
* check for name in workspace gaps command as well
A malformed command here will lead to the same result seen in #5131, so
add a check. Done inside the cmd_workspace_gaps() function itself, to
take advantage of the existing 'Expected...' string.
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