@vercel/blob - Return more useful error message when retrieveClientToken throws

As a user I should be able to know when retrieveClientToken failed for a specific reason. See the following example:

const jsonResponse = await handleUpload({
    body,
    request,
    onBeforeGenerateToken: async ( pathname: string, /* clientPayload?: string, */ ) => {
      // Generate a client token for the browser to upload the file

      // ⚠️ Authenticate users before generating the token.
      // Otherwise, you're allowing anonymous uploads.
      const profile = await getCurrentProfile();

      if (!profile) {
        console.log('User is not logged in');
        throw new Error('Not authenticated');
      }

      if (!userAccessControl.canUser(profile).createOwn(Resources.UPLOADS).granted) {
        console.log('User does not have permission to upload files');
        throw new Error('You do not have permission to upload files');
      }
      
      return {
        allowedContentTypes: ['application/pdf', 'text/plain'],
        maximumSizeInBytes: 8_000_000,
        tokenPayload: JSON.stringify({ profileId: profile.id })
      };
    },
    onUploadCompleted: async ({ blob, tokenPayload }) => {
      // ...
    }
});

Assuming the above is set up in a route to handle uploads, I should receive 'You do not have permission to upload files' when the user is authenticated but not permitted to perform uploads, instead of the current 'Failed to retrieve the client token' error.

It seems the library is already set up to return the error in the body to the client, so we could attempt to parse the body for the error message.

The relevant code is packages/blob/src/client.ts, at retrieveClientToken and the linked current error location; start by tracing the response body when token retrieval fails. Preserve the specific error text described in the issue instead of only the generic message, and verify the client receives “You do not have permission to upload files” for the shown callback failure.