Use BoxError instead of Error in all places

This moves everything other than the addon code and some 'done' logic

src/shell.js

@@ -7,6 +7,7 @@ exports = module.exports = {
 };
 
 var assert = require('assert'),
+    BoxError = require('./boxerror.js'),
     child_process = require('child_process'),
     debug = require('debug')('box:shell'),
     once = require('once'),
@@ -60,7 +61,7 @@ function spawn(tag, file, args, options, callback) {
         if (code || signal) debug(tag + ' code: %s, signal: %s', code, signal);
         if (code === 0) return callback(null);
 
-        var e = new Error(util.format(tag + ' exited with error %s signal %s', code, signal));
+        var e = new BoxError(BoxError.SPAWN_ERROR, `${tag} exited with error ${code} signal ${signal}`);
         e.code = code;
         e.signal = signal;
         callback(e);