Fixed error handling in less-pretty-cat plugin

When pygmentize is not installed, the functions less/cat are not
defined, the native commands will be used instead. Also removed the
error message that was shown when the command was not found.

Using `command -v` instead of `which` to check for the existence of the
executable - this works better as it does not print an error message if
the executable is not found.

plugins/available/less-pretty-cat.plugin.bash

@@ -1,18 +1,13 @@
 cite about-plugin
 about-plugin 'pygmentize instead of cat to terminal if possible'
 
-if [ -z $(which pygmentize) ]
-then
-    echo "Pygments is required to use this plugin"
-    echo "Install it by doing 'pip install Pygments' as the superuser"
-fi
-
+if $(command -v pygmentize &> /dev/null) ; then
   # get the full paths to binaries
   CAT_BIN=$(which cat)
   LESS_BIN=$(which less)
 
   # pigmentize cat and less outputs
-cat()
+  function cat()
   {
       about 'runs either pygmentize or cat on each file passed in'
       param '*: files to concatenate (as normally passed to cat)'
@@ -23,10 +18,11 @@ cat()
       done
   }
 
-less()
+  function less()
   {
       about 'it pigments the file passed in and passes it to less for pagination'
       param '$1: the file to paginate with less'
       example 'less mysite/manage.py'
       pygmentize "$*" | "$LESS_BIN" -R
   }
+fi