Added error handling for Assembly Converter. 

In my particular case, when there is no seq_region, it fails and shows an ajax error.
https://www.ebi.ac.uk/panda/jira/browse/ENSEMBL-3241
Now it can show a returned error.
Test scenario:
1. go to Tools > Assembly Converter
2. Copy& paste the below in the Paste data field.
Chr1 26346 26453 MACS_peak_1 36.18
Chr1 43809 43930 MACS_peak_2 25.28
Chr1 48424 48491 MACS_peak_3 32.64
3. Click submit.