Squashed commit of the following:

commit f93ec6430f9629b5a28765a3961379d1e8296e50
Author: Bob Nystrom <robert@stuffwithstuff.com>
Date:   Wed Mar 16 08:03:25 2016 -0700

    Tiny tweaks.

commit 332a08fe33cddffec82dbd377ee0c941a0066f64
Merge: a673981 14b7a08
Author: Bob Nystrom <robert@stuffwithstuff.com>
Date:   Wed Mar 16 08:01:00 2016 -0700

    Merge branch 'errorFn' of https://github.com/ppvk/wren into ppvk-errorFn

commit 14b7a086ce3dae1b12697319bc2894048fad469f
Author: Paul VanKeuren <paul.vankeuren@gmail.com>
Date:   Tue Mar 15 11:33:37 2016 -0500

    missing semicolon, accessing vm through parser in

commit 924eaa1f9d3fbb26dc07d3ac8be7dc00287138d9
Author: Paul VanKeuren <paul.vankeuren@gmail.com>
Date:   Tue Mar 15 11:25:32 2016 -0500

    fixed missing errorFn in wren.h

commit 3761df111f9dce3bd67407754675c9efbbf2c0b4
Author: Paul VanKeuren <paul.vankeuren@gmail.com>
Date:   Tue Mar 15 11:23:51 2016 -0500

    Added an errorFn to the CLI

commit d51137b4abe339a3f07d7e2f4c5ee0d8d062a009
Author: Paul VanKeuren <paul.vankeuren@gmail.com>
Date:   Tue Mar 15 11:21:16 2016 -0500

    Added a WrenErrorFn typedef, an errorFn slot in WrenConfiguration.  exits early if  is not defined, otherwise it runs the errorFn .

src/vm/wren_compiler.c

@@ -394,14 +394,16 @@ static void printError(Parser* parser, int line, const char* label,
   parser->hasError = true;
   if (!parser->printErrors) return;
   
+  // Only report errors if there is a WrenErrorFn to handle them.
+  if (parser->vm->config.errorFn == NULL) return;
+  
   // Format the label and message.
   char message[ERROR_MESSAGE_SIZE];
   int length = sprintf(message, "%s: ", label);
   length += vsprintf(message + length, format, args);
   ASSERT(length < ERROR_MESSAGE_SIZE, "Error should not exceed buffer.");
   
-  fprintf(stderr, "[%s line %d] %s\n", parser->module->name->value, line,
-          message);
+  parser->vm->config.errorFn(parser->module->name->value, line, message);
 }
 
 // Outputs a compile or syntax error. This also marks the compilation as having

src/vm/wren_vm.c

@@ -42,6 +42,7 @@ void wrenInitConfiguration(WrenConfiguration* config)
   config->bindForeignMethodFn = NULL;
   config->bindForeignClassFn = NULL;
   config->writeFn = NULL;
+  config->errorFn = NULL;
   config->initialHeapSize = 1024 * 1024 * 10;
   config->minHeapSize = 1024 * 1024;
   config->heapGrowthPercent = 50;