short subdomain error message formatting

[VladlenKaraush]

// input
jprq http 3000 -s fl // or any 1-2 character subdomain

//output
jprq 2.4         press Ctrl+C to quit

invalid subdomain [fl subdomain length must be between 3 and 42]: %!!(MISSING)s(MISSING)

also paincs on empty subdomain

// input 
jprq http 3000 -s

// output 
panic: runtime error: index out of range [1] with length 1

goroutine 1 [running]:
main.parseFlags(...)
        /Users/azimjon/Developer/opensource/jprq/cli/main.go:110
main.main()
        /Users/azimjon/Developer/opensource/jprq/cli/main.go:63 +0x7b8