Fix unhandled exception when target is unavailable

[StefH]

#467

[codecov]

Codecov Report

Merging #469 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #469   +/-   ##
=======================================
  Coverage   75.69%   75.69%           
=======================================
  Files         131      131           
  Lines        5044     5044           
  Branches      498      498           
=======================================
  Hits         3818     3818           
  Misses       1078     1078           
  Partials      148      148           

Impacted Files	Coverage Δ
src/WireMock.Net/Owin/WireMockMiddleware.cs	97.43% <ø> (ø)
src/WireMock.Net/Serialization/LogEntryMapper.cs	100.00% <ø> (ø)
src/WireMock.Net/Owin/AspNetCoreSelfHost.cs	85.54% <0.00%> (ø)
src/WireMock.Net/Matchers/CSharpCodeMatcher.cs	85.93% <0.00%> (ø)

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