Added own exception class and improved internal & user-level error handling. Cleaned up 404/error image loading

when loading up generated error & fallback images, set an image format to allow output without errors

fix normalisation of JPEG mimetype
use new exception class in Filters
make use of exception handling in request processor class
don't perform any generation on invalid or errored images, just pass them through as-is
updated readme
display error images at the exact resolution requested without applying any distorting operations

refactored retrieval of target dimensions
fix error in imthumb error header output
add method to clear params to 'unalter' state, fixes to fallback image loading
if image isn't valid when generating throw an exception

use new method