validations: support non-V8 browsers

Call `Error.captureStackTrace()` only when it is available.

Use `this.stack = (new Error).stack` when `captureStackTrace` is not
available but the `stack` property is (Firefox).

lib/validations.js

@@ -703,11 +703,21 @@ function ValidationError(obj) {
     messages: obj.errors
   };
 
+  if (Error.captureStackTrace) {
+    // V8 (Chrome, Opera, Node)
     Error.captureStackTrace(this, this.constructor);
+  } else if (errorHasStackProperty) {
+    // Firefox
+    this.stack = (new Error).stack;
+  }
+  // Safari and PhantomJS initializes `error.stack` on throw
+  // Internet Explorer does not support `error.stack`
 }
 
 util.inherits(ValidationError, Error);
 
+var errorHasStackProperty = !!(new Error).stack;
+
 function formatErrors(errors) {
   var DELIM = '; ';
   errors = errors || {};