Reject exec promise with hash instead of string

spec/stdlib/child-process-spec.coffee

@@ -88,6 +88,7 @@ describe 'Child Processes', ->
     describe "when the command fails", ->
       it "executes the callback with error set to the exit status", ->
         waitsForPromise shouldReject: true, ->
-          cmd = "exit 2"
+          cmd = "echo 'bad' >&2 && exit 2"
           ChildProcess.exec(cmd).fail (error) ->
             expect(error.exitStatus).toBe 2
+            expect(error.stderr).toBe "bad\n"

src/stdlib/child-process.coffee

@@ -15,7 +15,7 @@ class ChildProccess
 
     $native.exec command, options, (exitStatus, stdout, stderr) ->
       if exitStatus != 0
-        deferred.reject("Command '#{command}' failed with exit status #{exitStatus} and stderr '#{stderr}'")
+        deferred.reject({command, exitStatus, stderr})
       else
         deferred.resolve(stdout, stderr)