Support JSON as the default serialisation for errors. Bug found by Lel on dev
Service printed line breaks between multiple sequences to STDOUT and not to the file handle
Adding tests for multiple sequences and text/plain.

Checking that the changes will not regress by adding two new tests. The
firsts makes a bad request for multiple sequences and makes sure we
correctly error. The second makes sure we have 1 sequence per line.
Merge pull request #15 from Ensembl/hotfix/sequenceerrorbug

Hotfix/sequenceerrorbug