Handling "not found" and other IOExceptions differently in ParseableURL

Hi.

Recently I got some weird bugs because a web server that distributes configuration over parts of my system went crazy and kept responding 503 to some urls used in "include" statements in the system configuration. Corresponding exceptions were silently ignored and we ended up with the system running on default settings. 

It seems to me that errors which are essentially a "missing file" error, should be treated as they are now, but other types of IOExceptions should attract some user attention, because they are most probably an evidence of broken environment. 
Here is a patch for ParseableURL that do it.

Thanks for this awesome library,

Pavel.