fix(minErr): encode btstrpd error input to strip angle brackets

The $sanitize service was returning an empty string to the error page
because the input was usually a single html tag (sometimes it could be
`document`). This fix replaces angle brackets with html entities.

Closes #8683

Author: jeffbcross

src/Angular.js

@@ -1398,7 +1398,11 @@ function bootstrap(element, modules, config) {
 
     if (element.injector()) {
       var tag = (element[0] === document) ? 'document' : startingTag(element);
-      throw ngMinErr('btstrpd', "App Already Bootstrapped with this Element '{0}'", tag);
+      //Encode angle brackets to prevent input from being sanitized to empty string #8683
+      throw ngMinErr(
+          'btstrpd',
+          "App Already Bootstrapped with this Element '{0}'",
+          tag.replace(/</,'&lt;').replace(/>/,'&gt;'));
     }
 
     modules = modules || [];

test/AngularSpec.js

@@ -796,7 +796,7 @@ describe('angular', function() {
       expect(function () {
         angular.bootstrap(element);
       }).toThrowMatching(
-        /\[ng:btstrpd\] App Already Bootstrapped with this Element '<div class="?ng\-scope"?( ng[0-9]+="?[0-9]+"?)?>'/i
+        /\[ng:btstrpd\] App Already Bootstrapped with this Element '&lt;div class="?ng\-scope"?( ng[0-9]+="?[0-9]+"?)?&gt;'/i
       );
 
       dealoc(element);