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/cli/vm.c

@@ -156,6 +156,11 @@ static void write(WrenVM* vm, const char* text)
   fflush(stdout);
 }
 
+static void reportError(const char* module, int line, const char* message)
+{
+  fprintf(stderr, "[%s line %d] %s\n", module, line, message);
+}
+
 static void initVM()
 {
   WrenConfiguration config;
@@ -165,6 +170,7 @@ static void initVM()
   config.bindForeignClassFn = bindForeignClass;
   config.loadModuleFn = readModule;
   config.writeFn = write;
+  config.errorFn = reportError;
 
   // Since we're running in a standalone process, be generous with memory.
   config.initialHeapSize = 1024 * 1024 * 100;