Split out checkExecutePermissions() function from userCanExecute() in FormSpecialPage...

Split out checkExecutePermissions() function from userCanExecute() in FormSpecialPage. The former handles actual execute() calls and throws exceptions, the later inherits its purpose from SpecialPage. It just checks the page restriction to see if a user generally *could* execute. This avoids the breakage that came up in r100723 where Special:SpecialPages was throwing permission errors due to the getUsablePages() call.