Handle offset not found

[Gsantomaggio]

Add specific exception in case the offset does not exist
fix the ConsumerQueryOffset test.

[codecov-commenter]

Codecov Report

Merging #73 (995d4b1) into main (90c175d) will increase coverage by 0.10%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #73      +/-   ##
==========================================
+ Coverage   90.44%   90.54%   +0.10%     
==========================================
  Files          66       66              
  Lines        4386     4402      +16     
  Branches      258      260       +2     
==========================================
+ Hits         3967     3986      +19     
+ Misses        362      359       -3     
  Partials       57       57              

Impacted Files	Coverage Δ
RabbitMQ.Stream.Client/ClientExceptions.cs	61.29% <100.00%> (+5.73%)	⬆️
RabbitMQ.Stream.Client/StreamSystem.cs	90.47% <100.00%> (+0.09%)	⬆️
Tests/ConsumerSystemTests.cs	100.00% <100.00%> (ø)
Tests/Utils.cs	92.40% <100.00%> (+3.79%)	⬆️

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[lukebakken]

Nice!