rescue error when designated opener is not found

$ binman -h
/gems/opener-0.1.0/lib/opener.rb:97:in `spawn': No such file or directory - xdg-open (Errno::ENOENT)
        from /gems/opener-0.1.0/lib/opener.rb:97:in `spawn'
        from /gems/binman-3.3.1/lib/binman.rb:57:in `block in show'
        from /gems/binman-3.3.1/lib/binman.rb:55:in `each'
        from /gems/binman-3.3.1/lib/binman.rb:55:in `show'
        from /gems/binman-3.3.1/lib/binman.rb:88:in `help'
        from /gems/binman-3.3.1/bin/binman:106:in `<top (required)>'
        from /bin/binman:23:in `load'
        from /bin/binman:23:in `<main>'
exit 1
fix undefined local variable or method `library'

binman: undefined local variable or method `library' for BinMan:Module