Fix: propagate exceptions from slow_validate

[mdickinson]

If an exception other than TraitError occurs in a slow_validate call, it should be propagated rather than swallowed. Propagation is the usual behaviour, but it doesn't happen for code that goes through the validate_trait_complex code path. This PR fixes that.

The particular case that motivated this PR: given a Trait definition:

class A(HasTraits):
    foo = Either(None, Range(0, 100))

if the range validation check fails due to a bad __index__ method on the target object, for example, we would expect to see the exception from that method. Instead, it's currently swallowed.

[codecov-io]

Codecov Report

Merging #581 into master will increase coverage by 0.3%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master     #581     +/-   ##
=========================================
+ Coverage   65.85%   66.15%   +0.3%     
=========================================
  Files          44       44             
  Lines        6900     6900             
  Branches     1404     1404             
=========================================
+ Hits         4544     4565     +21     
+ Misses       1939     1922     -17     
+ Partials      417      413      -4

Impacted Files	Coverage Δ
traits/trait_types.py	64.48% <0%> (+0.88%)	⬆️
traits/etsconfig/etsconfig.py	63.58% <0%> (+6.17%)	⬆️

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[mdickinson]

Note: while this change could be regarded as a bugfix, it's got quite high potential for causing accidental breakage in real projects (even though any code it breaks is arguably itself buggy), so it wouldn't be appropriate to put this in a bugfix release. But I think it's okay for 6.0.