don't show exception if unknown protein accession encountered

[timosachsenberg]

Description

Fixes #5798 by eating the exception and hides the "show" sequence column if no webkit installed.

Checklist:

• Make sure that you are listed in the AUTHORS file
• Add relevant changes and new features to the CHANGELOG file
• I have commented my code, particularly in hard-to-understand areas
• New and existing unit tests pass locally with my changes
• Updated or added python bindings for changed or new classes. (Tick if no updates were necessary.)

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