Python generic exception handling

[mwm5945]

What this PR does / why we need it:
This fixes how exceptions are handled by the REST Python server. Previously, any unhandle-ed exceptions would result in a very generic Flask error, which is not parse-able into a seldon message. This adds a generic exception handler that returns a 500 error in a Seldon message. Custom error handlers inside the user object still function correctly.

Which issue(s) this PR fixes:

Fixes #2338

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Python microservice generic exception handling

[seldondev]

Hi @mwm5945. Thanks for your PR.

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[seldondev]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by:
To complete the pull request process, please assign ivan-valkov
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[mwm5945]

/assign @ivan-valkov

[mwm5945]

cc: @axsaucedo

[ukclivecox]

/ok-to-test

[ukclivecox]

/test integration