ssh/knownhosts: add file + linenumber for parse errors

Change-Id: Iddcb145ecd8a6b51c72ad3d77b242975baf4a5cf
Reviewed-on: https://go-review.googlesource.com/41210
Run-TryBot: Han-Wen Nienhuys <hanwen@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Sam Whited <sam@samwhited.com>
Reviewed-by: Han-Wen Nienhuys <hanwen@google.com>

ssh/knownhosts/knownhosts.go

@@ -401,7 +401,7 @@ func (db *hostKeyDB) Read(r io.Reader, filename string) error {
 		}
 
 		if err := db.parseLine(line, filename, lineNum); err != nil {
-			return err
+			return fmt.Errorf("knownhosts: %s:%d: %v", filename, lineNum, err)
 		}
 	}
 	return scanner.Err()