Adds a :vuln_test option to BES, just like in BAP.

I needed this to run a custom JS check for the Android
webview vuln when the exploit is served straight
through BES. The check already existed when using BAP,
so I tried to preserve that syntax, and also added a
:vuln_test_error as an optional error message.

This commit also does some mild refactoring of un-
useful behavior in BES.

modules/exploits/android/browser/webview_addjavascriptinterface.rb

@@ -12,18 +12,20 @@ class Metasploit3 < Msf::Exploit::Remote
   include Msf::Exploit::Remote::BrowserAutopwn
   include Msf::Exploit::Android
 
+  VULN_CHECK_JS = %Q|
+    for (i in top) {
+      try {
+        top[i].getClass().forName('java.lang.Runtime');
+        is_vuln = true; break;
+      } catch(e) {}
+    }
+  |
+
   autopwn_info(
     :os_flavor  => 'Android',
     :javascript => true,
     :rank       => ExcellentRanking,
-    :vuln_test  => %Q|
-      for (i in top) {
-        try {
-          top[i].getClass().forName('java.lang.Runtime');
-          is_vuln = true; break;
-        } catch(e) {}
-      }
-    |
+    :vuln_test  => VULN_CHECK_JS
   )
 
   def initialize(info = {})
@@ -71,7 +73,9 @@ class Metasploit3 < Msf::Exploit::Remote
       'DefaultTarget'       => 0,
       'BrowserRequirements' => {
         :source     => 'script',
-        :os_flavor  => 'Android'
+        :os_flavor  => 'Android',
+        :vuln_test  => VULN_CHECK_JS,
+        :vuln_test_error => 'No vulnerable Java objects were found in this web context.'
       }
     ))