Fix missing variable in exception handler

Previously, "cmd" was passed to a different method.

The reason this never came up earlier is because on Unix, if a file is
not in the PATH, a Aruba::LaunchError is raised much earlier.

One way this could've been triggered is: if the program file existed,
but wasn't accessible (e.g. EACCES error). That way it wouldn't fail the
`which` test, but would fail once it was attempted to be executed.

lib/aruba/processes/spawn_process.rb

@@ -83,7 +83,7 @@ def start
             sleep startup_wait_time
           end
         rescue ChildProcess::LaunchError => e
-          raise LaunchError, "It tried to start #{cmd}. " + e.message
+          raise LaunchError, "It tried to start #{commandline}. " + e.message
         end
 
         after_run

spec/aruba/spawn_process_spec.rb

@@ -56,5 +56,39 @@
 
       it { expect {process.start}.to raise_error Aruba::LaunchError }
     end
+
+    context "with a childprocess launch error" do
+      let(:child) { instance_double(ChildProcess::AbstractProcess) }
+      let(:command) { 'foo' }
+
+      before do
+        allow(ChildProcess).to receive(:build).and_return(child)
+
+        # STEP 1: Simlulate the program exists (but will fail to run, e.g. EACCESS?)
+        allow(Aruba.platform).to receive(:which).with(command, anything).and_return("/foo")
+
+        # STEP 2: Simulate the failure on Windows
+        allow(child).to receive(:start).and_raise(ChildProcess::LaunchError, "Foobar!")
+
+        # TODO: wrap the result of ChildProcess.build with a special Aruba
+        # class , so the remaining mocks below won't be needed
+        allow(child).to receive(:leader=)
+
+        io = instance_double(ChildProcess::AbstractIO)
+        allow(io).to receive(:stdout=)
+        allow(io).to receive(:stderr=)
+
+        allow(child).to receive(:io).and_return(io)
+
+        allow(child).to receive(:duplex=)
+        allow(child).to receive(:cwd=)
+
+        allow(child).to receive(:environment).and_return({})
+      end
+
+      it "reraises LaunchError as Aruba's LaunchError" do
+        expect { process.start }.to raise_error(Aruba::LaunchError, "It tried to start foo. Foobar!")
+      end
+    end
   end
 end