[MRG] Fix reporting of bad characters in input by reporting entire k-mer

[ctb]

Per #502 (comment), this fixes a problem where the wrong "bad" character was being reported out from --check-sequence.

The root of the problem was an bad attempt to figure out exactly which character was causing problems. This is a conservative fix that simply reports the whole k-mer that is causing problems, which should be sufficient.

Fixes #502.

• Is it mergeable?
• make test Did it pass the tests?
• make coverage Is the new code covered?
• Did it change the command-line interface? Only additions are allowed
  without a major version increment. Changing file formats also requires a
  major version number increment.
• Was a spellchecker run on the source code and documentation after
  changes were made?

[codecov-io]

Codecov Report

Merging #503 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #503      +/-   ##
==========================================
- Coverage   90.52%   90.51%   -0.01%     
==========================================
  Files          33       33              
  Lines        4937     4936       -1     
  Branches       36       36              
==========================================
- Hits         4469     4468       -1     
  Misses        467      467              
  Partials        1        1

Impacted Files	Coverage Δ
sourmash/kmer_min_hash.hh	99.35% <100%> (-0.01%)	⬇️

---

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 b0acc34...3b7e37b. Read the comment docs.