fmt: add Errorf helper function

This crops up in a lot of places.
It's just a one-liner, but doesn't add any dependancies.
Seems worth it.

R=r, r2
CC=golang-dev
https://golang.org/cl/2344041

src/pkg/fmt/print.go

@@ -159,6 +159,12 @@ func Sprintf(format string, a ...interface{}) string {
 	return s
 }
 
+// Errorf formats according to a format specifier and returns the string 
+// converted to an os.ErrorString, which satisfies the os.Error interface.
+func Errorf(format string, a ...interface{}) os.Error {
+	return os.ErrorString(Sprintf(format, a...))
+}
+
 // These routines do not take a format string
 
 // Fprint formats using the default formats for its operands and writes to w.