install: Improve error handling.
install: Improve error handling.

On usage errors (except --help), write usage message to stderr and
exit with nonzero status.

Forbid setting the hostname and email to the example values.  Those
are specifically checked for and would fail later.

Signed-off-by: Anders Kaseorg <087231934ba8fcf78bc142ce37d193b895ac03c4@mit.edu>

timabbott/zulip,zulip/zulip,rishig/zulip,andersk/zulip,timabbott/zulip,tommyip/zulip,rht/zulip,rht/zulip,tommyip/zulip,eeshangarg/zulip,andersk/zulip,andersk/zulip,andersk/zulip,eeshangarg/zulip,zulip/zulip,brainwane/zulip,synicalsyntax/zulip,zulip/zulip,eeshangarg/zulip,shubhamdhama/zulip,kou/zulip,kou/zulip,tommyip/zulip,punchagan/zulip,tommyip/zulip,brainwane/zulip,punchagan/zulip,shubhamdhama/zulip,hackerkid/zulip,zulip/zulip,showell/zulip,showell/zulip,shubhamdhama/zulip,rht/zulip,kou/zulip,shubhamdhama/zulip,brainwane/zulip,synicalsyntax/zulip,timabbott/zulip,rht/zulip,synicalsyntax/zulip,hackerkid/zulip,brainwane/zulip,zulip/zulip,timabbott/zulip,synicalsyntax/zulip,hackerkid/zulip,rishig/zulip,tommyip/zulip,punchagan/zulip,rishig/zulip,rht/zulip,rht/zulip,showell/zulip,timabbott/zulip,showell/zulip,rishig/zulip,andersk/zulip,hackerkid/zulip,shubhamdhama/zulip,timabbott/zulip,kou/zulip,andersk/zulip,punchagan/zulip,rishig/zulip,synicalsyntax/zulip,tommyip/zulip,kou/zulip,kou/zulip,rishig/zulip,hackerkid/zulip,eeshangarg/zulip,eeshangarg/zulip,hackerkid/zulip,rishig/zulip,eeshangarg/zulip,zulip/zulip,shubhamdhama/zulip,brainwane/zulip,punchagan/zulip,brainwane/zulip,brainwane/zulip,punchagan/zulip,showell/zulip,eeshangarg/zulip,hackerkid/zulip,rht/zulip,shubhamdhama/zulip,punchagan/zulip,zulip/zulip,showell/zulip,synicalsyntax/zulip,kou/zulip,showell/zulip,timabbott/zulip,andersk/zulip,synicalsyntax/zulip,tommyip/zulip