Consistent error behaviour on incomplete browse urls

Currently, typing in a valid browse URL with incomplete arguments (i.e. /browse/revision/ instead of /browse/revision/ec72c666fb345ea5f21359b7bc063710ce558e39/) returns an Internal Error - or a traceback in dev mode - instead of correctly informing the user that he has typed in a URL that lacks arguments.

We would like to review the browsing view's routes to correctly interpret the absence of arguments and inform the user with the appropriate http code.

Edit: this was in fact an artifact of leaving the argument-less routes in the code (i.e. /browse/content/). Removing these will cause flask to correctly return a 404 to the client in case of an argument-less URL.

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Migrated from T521 (view on Phabricator)