Upload Plugin from URL: Fix Error messages and UI

Description

This UX review has all the details on what needs to be changed: https://mattermost.atlassian.net/browse/MM-16926 . See comments area.

QA Test Steps

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Status

Mana

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Assignee

Unassigned

QA Assignee

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Reporter

Aaron Rothschild

Epic Link

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Fix versions

Mattermost Team

Toolkit

Sprint

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Labels

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QA Testing Areas

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GitHub Issue

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Components

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