Indent multiline failure messages in the output of withContext

This makes it easier to see where each failure message begins and ends.

Before:
   Some context: a
   multiline
   message

After:
   Some context:
       a
       multiline
       message

src/core/Expectation.js

@@ -176,9 +176,19 @@ getJasmineRequireObj().Expectation = function(j$) {
   }
 
   ContextAddingFilter.prototype.modifyFailureMessage = function(msg) {
-    return this.message + ': ' + msg;
+    var nl = msg.indexOf('\n');
+
+    if (nl === -1) {
+      return this.message + ': ' + msg;
+    } else {
+      return this.message + ':\n' + indent(msg);
+    }
   };
 
+  function indent(s) {
+    return s.replace(/^/gm, '    ');
+  }
+
   return {
     factory: function(options) {
       return new Expectation(options || {});