Soong: Add build_error proto raw file to $(OUT_DIR).

Soong_ui produces a build_error raw file to $(OUT_DIR) where
the file contains a list of build action errors. Each build action
error represents an error raised by a command. The build_error file
is populated if there was a build error.

Bug: b/132969697
Test: Ran m for successful build. Introduced a broken build change,
      ran m and verified using printproto that build_error was
      generated successfully.

Change-Id: I690ca1778b4e56f144a3173ba1d16d8494c32c15
diff --git a/cmd/soong_ui/main.go b/cmd/soong_ui/main.go
index b63918f..d3ef415 100644
--- a/cmd/soong_ui/main.go
+++ b/cmd/soong_ui/main.go
@@ -161,6 +161,7 @@
 	trace.SetOutput(filepath.Join(logsDir, "build.trace"))
 	stat.AddOutput(status.NewVerboseLog(log, filepath.Join(logsDir, "verbose.log")))
 	stat.AddOutput(status.NewErrorLog(log, filepath.Join(logsDir, "error.log")))
+	stat.AddOutput(status.NewProtoErrorLog(log, filepath.Join(logsDir, "build_error")))
 
 	defer met.Dump(filepath.Join(logsDir, "soong_metrics"))