[MRG] fix Rust panic error in signature creation

[ctb]

Fixes #1166 by adjusting rust code to raise proper error on add_protein with a DNA hash function.

• 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]

Codecov Report

Merging #1172 into latest will not change coverage.
The diff coverage is 50.00%.

@@           Coverage Diff           @@
##           latest    #1172   +/-   ##
=======================================
  Coverage   84.05%   84.05%           
=======================================
  Files         100      100           
  Lines        9300     9300           
=======================================
  Hits         7817     7817           
  Misses       1483     1483           

Flag	Coverage Δ
#rusttests	67.86% <50.00%> (ø)

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Impacted Files	Coverage Δ
src/core/src/signature.rs	54.40% <50.00%> (ø)

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