Raise an error during capture if CutyCapt is not installed

[henrytwang]

If CutyCapt is not installed and I attempt to capture a webpage, the current error is cryptic:

sh: --: invalid option
Usage:  sh [GNU long option] [option] ...
  sh [GNU long option] [option] script-file ...
GNU long options:
  --debug
  --debugger
  --dump-po-strings
  --dump-strings
  --help
  --init-file
  --login
  --noediting
  --noprofile
  --norc
  --posix
  --protected
  --rcfile
  --restricted
  --verbose
  --version
  --wordexp
Shell options:
  -irsD or -c command or -O shopt_option    (invocation only)
  -abefhkmnptuvxBCHP or -o option

I added a friendly error message instead:

 RuntimeError: Cutycapt is not installed.