FIX: clear stale _ex before each execute_stream call

[stvoutsin]

Description

_ex was never cleared between execute_stream calls, so there was the potential that a stale HTTP error from a previous failed request could mask VOTable parse errors on a later successful one.

Fix

Reset self._ex = None at the start of execute_stream

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.57%. Comparing base (f14fabb) to head (b3910bf).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #751      +/-   ##
==========================================
+ Coverage   79.54%   79.57%   +0.02%     
==========================================
  Files          91       91              
  Lines       10293    10294       +1     
==========================================
+ Hits         8188     8191       +3     
+ Misses       2105     2103       -2     

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[msdemlei]

Thanks!