Necessary to preserve type information.  The try/catch sequence didn't
work previously.

Apparently, the std::system_error constructor appends strerror()
already.

This compatibility hack was accidently broken by commit f072cbbb

memcmp() can overflow the buffer.

Regression by commit 738583e3

The state ERROR can only be entered during decoder initialization, and
Seek() may only be called after initialization has been finished.