function prints the given usage error message to stderr and terminates
execution of the script with a return value of 1.
The message can be provided as one or more arguments as depicted by
all arguments are concatenated together into a single line.
The printed message has the following form, where
is determined by
progname: E: message1 .. messageN
Type man progname for help.
should be used to inform the user of problems directly caused by his invocation
of the script (e.g. the use of an invalid flag).
Note that the printed error message refers to a manual page for the script,
so you should make sure to provide one.