caddy fmt overwrite should emit an error message

[bt90]

Currently caddy fmt --overwrite fails rather silently if the file couldn't be modified:

user@vm:$ caddy fmt --overwrite /etc/caddy/Caddyfile
user@vm:$ echo $?
1

Could we add an error message? This would also follow other unix tools:

user@vm:$ mkdir /data
mkdir: cannot create directory '/data': Operation not permitted

[mholt]

Good point, we don't print the error message currently. We should do that.