Clarify error in AddIpoptIntOption

[odow]

Closes #323

@this-josh is this better?

[codecov[bot]]

Codecov Report

Merging #325 (cddff43) into master (e5f978c) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #325   +/-   ##
=======================================
  Coverage   87.80%   87.80%           
=======================================
  Files           3        3           
  Lines         697      697           
=======================================
  Hits          612      612           
  Misses         85       85           

Impacted Files	Coverage Δ
src/C_wrapper.jl	86.95% <100.00%> (ø)

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