Add exception to expose existing (persistent) subscription response

[vtermanis]

• Distinguish creation failure from already-existing state

[codecov]

Codecov Report

Merging #137 into master will decrease coverage by 0.06%.
The diff coverage is 75%.

@@            Coverage Diff             @@
##           master     #137      +/-   ##
==========================================
- Coverage   85.31%   85.25%   -0.07%     
==========================================
  Files           8        8              
  Lines        2254     2258       +4     
  Branches      238      239       +1     
==========================================
+ Hits         1923     1925       +2     
- Misses        275      276       +1     
- Partials       56       57       +1

Impacted Files	Coverage Δ
photonpump/exceptions.py	91.3% <100%> (+0.25%)	⬆️
photonpump/conversations.py	95.98% <50%> (-0.28%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 25e83f7...2aa6884. Read the comment docs.

[vtermanis]

@bobthemighty I probably won't have time to make additional changes here if required before next Wednesday so please do feel free to amend where necessary (as well as in the other PR). I presumse codecov "fails" because there's no test case for actual subscription failure (simulation) and hence that branch is no longer exercised.