[errors] Greatly simplify common/errors package.

vpython/venv/system_windows.go

Index: vpython/venv/system_windows.go
diff --git a/vpython/venv/system_windows.go b/vpython/venv/system_windows.go
index 81e17673a86b40b3000e966551af1a6627b38c5c..aa128c7d62f87c04c62c535e4695f457ab42c859 100644
--- a/vpython/venv/system_windows.go
+++ b/vpython/venv/system_windows.go
@@ -26,11 +26,11 @@ func venvBinDir(root string) string {
 func checkProcessRunning(pid int) error {
 	proc, err := os.FindProcess(pid)
 	if err != nil {
-		return errors.Annotate(err).Reason("failed to find process").Err()
+		return errors.Annotate(err, "failed to find process").Err()
 	}
 
 	if err := proc.Signal(os.Signal(syscall.Signal(0))); err != nil {
-		return errors.Annotate(err).Reason("failed to signal process").Err()
+		return errors.Annotate(err, "failed to signal process").Err()
 	}
 	return nil
 }