Make error alert accept error dictionary and default error

What does this MR do and why?

Make friendly error alert in purchase flow accept error dictionary and default error to make it reusable

• Makes error_alert.vue reusable from outside purchase flows by allowing to specify specific error dictionary and default error message
• Updates the utils to be pure functions for re-usability
• Moves error_alert.vue up a level to make it reusable from outside purchase flows, eg. usage quotas
• Moves error constants to a separate file

We'll be using the updated error_alert.vue component in this MR from usage quotas page.

Screenshots or screen recordings

No changes to the UI, corresponding friendly errors should be displayed as before.

Scenario	Before	After
Error message as string	Plan Purchase flow AddOn Purchase flow	Plan Purchase flow AddOn Purchase flow
Error with attribute map	Plan Purchase flow AddOn Purchase flow	Plan Purchase flow AddOn Purchase flow
Error message as object	Plan Purchase flow AddOn Purchase flow	Plan Purchase flow AddOn Purchase flow
Error with no message	Plan Purchase flow AddOn Purchase flow	Plan Purchase flow AddOn Purchase flow
Error with no matching friendly error	Plan Purchase flow AddOn Purchase flow	Plan Purchase flow AddOn Purchase flow

How to set up and validate locally

1. Customers Dot needs to be setup in order to view the billing page for a group
2. Start GDK in SaaS mode GITLAB_SIMULATE_SAAS=1 gdk start
3. Login into gdk
4. Go to a group where you are an owner (or create one)
5. Copy the group id
6. Plan purchase flow: Visit <your-gdk-url>/-/subscriptions/new?namespace_id=<your-group-id>&plan_id=2c92a00d76f0d5060176f2fb0a5029ff
7. AddOn Purchase Flow: Visit <your-gdk-url>/-/subscriptions/buy_minutes?selected_group=<your-group-id>
8. The best way to see the error as soon as you land on the page is to use this patch: error_mapping_patch.patch
   • You can change the constant in new Error(EMAIL_TAKEN_ERROR) to
     • EMAIL_TAKEN_ERROR_TYPE (for attribute map)
     • FIRST_NAME_BLANK_ERROR_VARIATION (for error message as object)
     • empty (new Error() for no message)
     • a random string new Error('no matching friendly error')) for no mapping available scenario.

Storybook

1. Start storybook yarn storybook:start
2. Visit http://localhost:9002/?path=/story/ee-vue-shared-components-error-alert--default

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #420010