Sets the named function as the current error handler, or unsets
the current error handler if
NULL. The error-handler function is
called whenever an error occurs; the function can do whatever it
wants, but typically will print an error message and clean up after a
critical error happens.
The user-defined function is called with two parameters, an error code and a string describing the error. Three additional parameters may also be supplied—the filename in which the error occurred, the line number at which the error occurred, and the context in which the error occurred (which is an array pointing to the active symbol table).
the name of the previously installed error-handler function, or
false if an error occurred while
setting the error handler (e.g., when
function doesn’t exist).