ZF-2528: Zend_Form __toString() implementations should not throw exceptions

Description

Currently, all __toString() implementations in the Zend_Form classes proxy to render(), which in turn proxies to decorators and potentially other classes -- any of which could throw an exception. PHP doesn't like exceptions in __toString() and throws an E_FATAL that doesn't accurately depict the root cause of the issue.

To make debugging simpler, __toString() should capture exceptions, and then raise an E_USER_WARNING with the exception message.

Comments

Resolved in r7697