Graceful error if serialport gem is needed.

tools/hardware/elm327_relay.rb

@@ -10,12 +10,20 @@
 ####
 # This module requires a connected ELM327 or STN1100 is connected to
 # the machines serial.  Sets up a basic RESTful web server to communicate
-# Requires MSF and the serialport gem
+#
+# Requires MSF and the serialport gem to be installed.
+# - `gem install serialport`
+# - or, if using rvm: `rvm gemset install serialport`
 ####
 
 
 ### Non-typical gem ###
-require 'serialport'
+begin
+  require 'serialport'
+rescue LoadError => e
+  gem = e.message.split.last
+  abort "#{gem} is not installed. Please install with `gem install #{gem}' or, if using rvm, `rvm gemset install #{gem}' and try again."
+end
 
 #
 # Load our MSF API