[errors] Greatly simplify common/errors package.

vpython/venv/system_posix.go

Index: vpython/venv/system_posix.go
diff --git a/vpython/venv/system_posix.go b/vpython/venv/system_posix.go
index de3e8e2e85c3f7a65dc5b51d38f56ff68beceda7..3d78d449090c3a1efde1c79ebbb998ce6e5bd753 100644
--- a/vpython/venv/system_posix.go
+++ b/vpython/venv/system_posix.go
@@ -28,11 +28,11 @@ 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
 }