fix: Return false when process event guard fails

[erikzenker]

No description provided.

[codecov-commenter]

Codecov Report

Merging #174 (f98c0cd) into master (fbea52e) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #174      +/-   ##
==========================================
+ Coverage   97.74%   97.76%   +0.01%     
==========================================
  Files          78       79       +1     
  Lines        1111     1118       +7     
==========================================
+ Hits         1086     1093       +7     
  Misses         25       25              

Impacted Files	Coverage Δ
test/integration/unexpected_event_handler.cpp	100.00% <ø> (ø)
include/hsm/details/sm.h	100.00% <100.00%> (ø)
test/integration/process_event_return_bool.cpp	100.00% <100.00%> (ø)
include/hsm/details/variant_queue.h	81.81% <0.00%> (-2.80%)	⬇️

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