Bug 19417: Display full error text when problem importing records

Bug 19417: Display full error text when problem importing records

To test:
1 - Upload a file to stage
2 - Select your import options
3 - Delete the file from the server, example:
    sudo rm /tmp/koha_kohadev_uploads/
4 - 'Click stage for import'
5 - Get an alert 'Failed to submit form: error'
6 - Apply patches
7 - Repeat 1-4
8 - Get a pretty error with additional info

Signed-off-by: Liz Rea <wizzyrea@gmail.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>