in SHELL-COMMAND-TO-STRING, throw an error upward

We want to treat all errors in SHELL-COMMAND-TO-STRING as fatal unless
the caller installs a handler for ERROR-IN-SHELL-COMMAND-TO-STRING.

ffmpeg-myth.scm

@@ -59,7 +59,8 @@
 	str))
     ;; handler
     (lambda (key cmd str)
-      (simple-format #t "ERROR: in command ~a\nstring: ~a\n" cmd str))))
+      (simple-format #t "ERROR: in command ~a\nstring: ~a\n" cmd str)
+      (throw 'error-in-shell-command-to-string cmd str))))
 
 (define (mysql-escape-quote str)
   ;;  (string-join (string-split str #\') "'\"'\"'"))