Fix making kernel dumps from the debugger by creating a command
for it. Do not not expect a developer to call doadump(). Calling
doadump does not necessarily work when it's declared static. Nor
does it necessarily do what was intended in the context of text
dumps. The dump command always creates a core dump.

Move printing of error messages from doadump to the dump command,
now that we don't have to worry about being called from DDB.