Report "Parsing redeem response failed" better

Fixes #102 (closed)

The logs for this case will now look something like this:

[_zkapauthorizer.controller.PaymentController#error] Redeeming random tokens for a voucher (dnP...) failed: UnexpectedResponse(code=500, body='exception: SQLite3 returned ErrorBusy while attempting to perform step: database is locked') 

The underlying problem is unresolved, of course, but at least it should be more clear which problem is being hit.

Created by: codecov[bot]

Codecov Report

Merging #168 into master will increase coverage by 0.09%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #168      +/-   ##
==========================================
+ Coverage   96.74%   96.84%   +0.09%     
==========================================
  Files          43       43              
  Lines        3498     3513      +15     
  Branches      455      457       +2     
==========================================
+ Hits         3384     3402      +18     
+ Misses         77       74       -3     
  Partials       37       37              

Impacted Files	Coverage Δ
src/_zkapauthorizer/controller.py	94.42% <100.00%> (+1.07%)
src/_zkapauthorizer/tests/test_controller.py	98.10% <100.00%> (+0.08%)

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