Handle missing target vocabulary for update_vocab

[guillaumekln]

Fix #499.

[codecov-io]

Codecov Report

Merging #502 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #502   +/-   ##
=======================================
  Coverage   64.68%   64.68%           
=======================================
  Files          77       77           
  Lines        7169     7169           
=======================================
  Hits         4637     4637           
  Misses       2532     2532

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 228e697...b466176. Read the comment docs.