Bugfix for Inconsistent Alert Icon

[ctrimm]

Fixes #1425.

Changes the alert to utilize the error icon instead of the ban icon. This makes it consistent with the lightning system design.

I read thru the CONTRIBUTING.md file @interactivellama, but am a bit confused if I should check off the item in the Pull Request Review checklist below. Let me know and I can address this at a later date.

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CONTRIBUTOR checklist (do not remove)

Please complete for every pull request

• npm run lint:fix has been run and linting passes.
• Mocha, Jest (Storyshots), and components/component-docs.json CI tests pass (npm test).
• Tests have been added for new props to prevent regressions in the future. See readme.
• Review the appropriate Storybook stories. Open http://localhost:9001/.
• The Accessibility panel of each Storybook story has 0 violations (aXe). Open http://localhost:9001/.
• Review tests are passing in the browser. Open http://localhost:8001/.
• Review markup conforms to SLDS by looking at DOM snapshot strings.

REVIEWER checklist (do not remove)

• TravisCI tests pass. This includes linting, Mocha, Jest, Storyshots, and components/component-docs.json tests.
• Tests have been added for new props to prevent regressions in the future. See readme.
• Review the appropriate Storybook stories. Open http://localhost:9001/.
• The Accessibility panel of each Storybook story has 0 violations (aXe). Open http://localhost:9001/.
• Review tests are passing in the browser. Open http://localhost:8001/.
• Review markup conforms to SLDS by looking at DOM snapshot strings.
• Add year-first date and commit SHA to last-slds-markup-review in package.json and push.
• Request a review of the deployed Heroku app by the Salesforce UX Accessibility Team.
• Add year-first review date, and commit SHA, last-accessibility-review, to package.json and push.
• While the contributor's branch is checked out, run npm run local-update within locally cloned site repo to confirm the site will function correctly at the next release.

[salesforce-cla]

Thanks for the contribution! Before we can merge this, we need @ctrimm to sign the Salesforce.com Contributor License Agreement.

[interactivellama]

@ctrimm Thanks for the callout. Updated. https://github.com/salesforce/design-system-react/blob/master/docs/PULL_REQUEST_TEMPLATE.md

[interactivellama]

Please sign the CLA and I can merge.

[ctrimm]

@interactivellama - I signed the CLA on Tuesday. Thanks for the update to the PR Template - it looks much better now.