translate: Fix error handling of reference sequence features

[victorlin]

Description of proposed changes

Previously, the error message was never reached because len(features) would raise another error:

TypeError: object of type 'NoneType' has no len()

Switching the order allows for the intended error handling to happen.

Related issue(s)

Fixes uncaught error in #1151.

Testing

No testing as I'm not sure how to reproduce this scenario, but I think this is a trivial change.

Checklist

• Add a message in CHANGES.md summarizing the changes in this PR that are end user focused. Keep headers and formatting consistent with the rest of the file.

[codecov]

Codecov Report

Base: 68.17% // Head: 68.17% // No change to project coverage 👍

Coverage data is based on head (f192902) compared to base (6c9a901).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1168   +/-   ##
=======================================
  Coverage   68.17%   68.17%           
=======================================
  Files          63       63           
  Lines        6752     6752           
  Branches     1654     1654           
=======================================
  Hits         4603     4603           
  Misses       1841     1841           
  Partials      308      308           

Impacted Files	Coverage Δ
augur/translate.py	74.60% <100.00%> (ø)

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