The DbEnv::set_errcall method is used to replace the mechanism for
reporting error messages to the user. By default, DB writes error
messages using the C library's stderr. To allow alternative styles of
error reporting, set_errcall can be called with a single function pointer
argument. The function must be defined with two arguments; the first will
be the prefix string (set by set_errpfx), the second will be the error
message string. It is up to this function to display the message in an
appropriate manner.