command/build: put build errors in machine readable output

CHANGELOG.md

@@ -14,6 +14,7 @@ IMPROVEMENTS:
 
 * core: Output message when Ctrl-C received that we're cleaning up. [GH-338]
 * builder/amazon: Tagging now works with all amazon builder types.
+* command/build: Machine-readable output now contains build errors, if any.
 
 BUG FIXES:
 

command/build/command.go

@@ -187,8 +187,10 @@ func (c Command) Run(env packer.Environment, args []string) int {
 	}
 
 	if len(errors) > 0 {
+		env.Ui().Machine("error-count", strconv.FormatInt(int64(len(errors)), 10))
 		env.Ui().Error("\n==> Some builds didn't complete successfully and had errors:")
 		for name, err := range errors {
+			env.Ui().Machine("error", name, err.Error())
 			env.Ui().Error(fmt.Sprintf("--> %s: %s", name, err))
 		}
 	}

website/source/docs/machine-readable/command-build.html.markdown

@@ -39,7 +39,9 @@ of `packer build`.
 	<dt>artifact-count (1)</dt>
 	<dd>
 		<p>
-		The number of artifacts associated with the given target.
+		The number of artifacts associated with the given target. This
+		will always be outputted _before_ any other artifact information,
+		so you're able to know how many upcoming artifacts to look for.
 		</p>
 
 		<p>
@@ -122,4 +124,32 @@ of `packer build`.
 		<strong>Data 1: string</strong> - The string output for the artifact.
 		</p>
 	</dd>
+
+	<dt>error-count (1)</dt>
+	<dd>
+		<p>
+		The number of errors that occurred during the build. This will
+		always be outputted before any errors so you know how many are coming.
+		</p>
+
+		<p>
+		<strong>Data 1: count</strong> - The number of build errors as
+		a base 10 integer.
+		</p>
+	</dd>
+
+	<dt>error (2)</dt>
+	<dd>
+		<p>
+		A build error that occurred.
+		</p>
+
+		<p>
+		<strong>Data 1: name</strong> - The name of the build that had an error.
+		</p>
+
+		<p>
+		<strong>Data 2: error</strong> - The error message as a string.
+		</p>
+	</dd>
 </dl>