Description:
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currently you can throw an exception from the autoloader, hence if you can't 
find a class, your application can gracefully exists, instead of exiting via the 
class not found fatal error.
my problem is, that I would like to use multiple autoloader (for example in a 
project which uses multiple component, or framework), but in this case, I can't 
throw an Exception from my autoloader, because maybe the other autoloaders could 
load the class.
if I'm sure that I will register the last autoloader, the this isn't a 
problem(my last autoloader will throw the Exception on missing class), but maybe 
I have to load a component late of the exection.
it would be cool, if I could somehow register a callback which will be called, 
when the spl autoload callbacks was executed, but the class couldn't be loaded
of course, from this callback, I cannot continue the normal execution flow, 
however I can log the error, and let the callback return and get the class not 
found fatal error, or throw an Exception, and handle/log the error from a higher 
level.