Return the reason for a validation failure

[andamian]

Exceptions due to attribute invalid values are caught and replaced with an InvalidArgumentException with the message "invalid input: /". This message is passed to the BadRequest Http error and it's the only indication that the users gets making it difficult for them to diagnose the cause.

[pdowler]

I will need an example in order to diagnose and fix.

[SharonGoliath]

There is an example in my home directory Cdemo_ext2_SCIRED.fits.xml. github claims it's empty, or it would be here.