backtrans-align breaks if any dna seq length isn't a multiple of 3

[metasoarous]

We get the following error message and stack trace:

% seqmagick backtrans-align sorted_aligned_inseqs_translation.fasta sorted_inseqs.fasta -o test.fa                                                                                                                  17-01-20 - 12:59:13
Traceback (most recent call last):
  File "/app/seqmagick/0.6.0/bin/seqmagick", line 9, in <module>
    load_entry_point('seqmagick==0.6.0', 'console_scripts', 'seqmagick')()
  ...
  File "/app/seqmagick/0.6.0/lib/python2.7/site-packages/seqmagick-0.6.0-py2.7.egg/seqmagick/subcommands/backtrans_align.py", line 108, in <genexpr>
    for codon in codons)
  File "/app/python2/2.7.8/lib/python2.7/site-packages/Bio/Data/CodonTable.py", line 304, in get
    return self.__getitem__(codon)
  ...
  File "/app/python2/2.7.8/lib/python2.7/site-packages/Bio/Data/CodonTable.py", line 185, in list_possible_proteins
    c1, c2, c3 = codon
ValueError: need more than 1 value to unpack

I can confirm that when I cut off the extra nucleotides, the back translation operates without error.

At the very least, it would seem to be worth presenting a more helpful error message. Better yet though, we could either do this trimming automatically (with a warning), or add a flag for triggering this behavior.