CheckErr prints a user friendly error to STDERR and exits with a non-zero exit code.

DefaultSubCommandRun prints a command's help string to the specified output if no arguments (sub-commands) are provided, or a usage error otherwise.

MultipleErrors returns a newline delimited string containing the prefix and referenced errors in standard form.

RequireNoArguments exits with a usage error if extra arguments are provided.

DefaultPluginHandler implements PluginHandler.