Check aggregate exception during startup

[StefH]

No description provided.

[codecov]

Codecov Report

Merging #235 into master will decrease coverage by 0.15%.
The diff coverage is 52.63%.

@@            Coverage Diff             @@
##           master     #235      +/-   ##
==========================================
- Coverage   89.64%   89.49%   -0.16%     
==========================================
  Files         100      101       +1     
  Lines        4924     4939      +15     
==========================================
+ Hits         4414     4420       +6     
- Misses        510      519       +9

Impacted Files	Coverage Δ
src/WireMock.Net/Exceptions/WireMockException.cs	0% <0%> (ø)
src/WireMock.Net/Server/FluentMockServer.cs	91.43% <100%> (+0.16%)	⬆️

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