code: protected function chainThrowMethods()
      {
          $code  = "    function throwOn(\$method, \$exception = false, \$args = false) {\n";
          $code .= $this->bailOutIfNotMocked('$method');
          $code .= "        \$this->mock->throwOn(\$method, \$exception, \$args);\n";
          $code .= "    }\n";
          $code .= "    function throwAt(\$timing, \$method, \$exception = false, \$args = false) {\n";
          $code .= $this->bailOutIfNotMocked('$method');
          $code .= "        \$this->mock->throwAt(\$timing, \$method, \$exception, \$args);\n";
          $code .= "    }\n";
          $code .= "    function errorOn(\$method, \$error = 'A mock error', \$args = false, \$severity = E_USER_ERROR) {\n";
          $code .= $this->bailOutIfNotMocked('$method');
          $code .= "        \$this->mock->errorOn(\$method, \$error, \$args, \$severity);\n";
          $code .= "    }\n";
          $code .= "    function errorAt(\$timing, \$method, \$error = 'A mock error', \$args = false, \$severity = E_USER_ERROR) {\n";
          $code .= $this->bailOutIfNotMocked('$method');
          $code .= "        \$this->mock->errorAt(\$timing, \$method, \$error, \$args, \$severity);\n";
          $code .= "    }\n";
  
          return $code;
      }
docs: Adds code for chaining the throw methods.
  
  @return string Code for chains.