birth: Include pos and window in lexing error

Analog to prelude.js's `Parser#_error'.

* build-aux/bootstrap/birth.scm (parse-error): Add procedure.
  (lex): Use it for string error.

build-aux/bootstrap/birth.scm

@@ -85,7 +85,8 @@
             ;; quotes, which can be escaped with a backslash
             (("\"") (let ((str (js:match-regexp "^\"(|.*?[^\\\\])\""
                                                 trim)))
-                      (or str (error "missing closing string delimiter" str))
+                      (or str (parse-error
+                               src pos "missing closing string delimiter"))
                       ;; a string token consists of the entire string
                       ;; including quotes as its lexeme, but its value will
                       ;; be the value of the string without quotes due to
@@ -100,6 +101,17 @@
                (token "symbol" symbol trim newpos))))))))
 
 
+;; Throw an error with a window of surrounding source code.
+;;
+;; The "window" is simply ten characters to the left and right of the
+;; first character of the source input SRC that resulted in the error.
+;; It's a little more than useless.
+(define (parse-error src pos msg)
+  (let ((window (substring src (- pos 10) (+ pos 10))))
+    (error (string-append msg " (pos " pos "): " window)
+           src)))
+
+
 ;; Produce a token and recurse.
 ;;
 ;; The token will be concatenated with the result of the mutually