display error when process doesnt exist and is run by name

lib/foreman/engine.rb

@@ -41,6 +41,7 @@ class Foreman::Engine
   end
 
   def execute(name)
+    error "no such process: #{name}" unless procfile[name]
     fork procfile[name]
 
     trap("TERM") { puts "SIGTERM received"; terminate_gracefully }

spec/foreman/engine_spec.rb

@@ -48,6 +48,12 @@ describe "Foreman::Engine" do
       mock(subject).watch_for_termination
       subject.execute("alpha")
     end
+
+    it "shows an error running a process that doesnt exist" do
+      write_procfile
+      mock(subject).puts("ERROR: no such process: foo")
+      lambda { subject.execute("foo") }.should raise_error(SystemExit)
+    end
   end
 
   describe "environment" do