Update unauthenticated session loading to throw specific SessionNotFoundError

[paulspringett]

WHY are these changes introduced?

It's currently awkward to catch this specific error, requiring matching on the error message string.

Before

import {unauthenticated} from '~/shopify.server';

try {
  const {admin} = await unauthenticated.admin(shop);
} catch (err) {
  if (err.message.includes('Could not find a session for shop')) {
    // do something specific
  }
}

After

import {SessionNotFoundError} from '@shopify/shopify-app-remix';
import {unauthenticated} from '~/shopify.server';

try {
  const {admin} = await unauthenticated.admin(shop);
} catch (err) {
  if (err instanceof SessionNotFoundError) {
    // do something specific
  }
}

WHAT is this pull request doing?

Defines and exports a new error class extending ShopifyError, meaning it should be backwards compatible for anyone already doing err instanceof ShopifyError.

Throws SessionNotFoundError instead of ShopifyError if the session is missing.

Type of change

• Patch: Bug (non-breaking change which fixes an issue)
• Minor: New feature (non-breaking change which adds functionality)
• Major: Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• I have used yarn changeset to create a draft changelog entry (do NOT update the CHANGELOG.md files manually)
• I have added/updated tests for this change
• I have documented new APIs/updated the documentation for modified APIs (for public APIs)

[paulomarg]

🔥

Can't see any other places where we might need to call this, either!