Add exited status to SessionFlusher and rescue ThreadError

[sl0thentr0py]

If we are unable to create a Thread, the underlying process is probably in an aborting state, so just gracefully stop trying to do session logic.
Additionally, if we call session_flusher.kill, we don't want to run session logic from then on.

fixes #1848

[codecov-commenter]

Codecov Report

Merging #1851 (1092411) into master (e55cbf1) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1851      +/-   ##
==========================================
+ Coverage   98.38%   98.40%   +0.01%     
==========================================
  Files         146      146              
  Lines        8744     8773      +29     
==========================================
+ Hits         8603     8633      +30     
+ Misses        141      140       -1     

Impacted Files	Coverage Δ
sentry-ruby/lib/sentry/session_flusher.rb	98.03% <100.00%> (+0.21%)	⬆️
sentry-ruby/spec/sentry/session_flusher_spec.rb	100.00% <100.00%> (ø)
sentry-resque/lib/sentry/resque.rb	100.00% <0.00%> (+2.85%)	⬆️

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