Promises should be rejected with Error instances

[tduyduc]

Issue Description

Promise rejections in the source code are currently using (primitive) strings. For example:

This would make exception handling more cumbersome (because an additional type check is needed) and more difficult (all stack trace information is destroyed).

Rationale / Further Reading

• no-throw-literal ESLint rule

Example

import AppleAuth from 'apple-auth';

declare var auth: AppleAuth;
try {
  await auth.accessToken(code);
} catch (error) {
  typeof error; // Actual: "string" here. Doesn't have any stack trace or `message` property from a standard Error.
}

try {
  null.x;
} catch (error) {
  error instanceof Error; // Expected: an instance of Error (or a subclass).
}

Proposed Solution

All exceptions from Promise constructors should be instances of Error. Using the example of src/apple-auth.js from the above image:

 reject(
-  `AppleAuth Error - An error occurred while getting response from Apple's servers: 
-  ${response}${responseData ? (" | " + responseData) : ""}`
+  new Error(
+    `AppleAuth Error - An error occurred while getting response from Apple's servers: 
+    ${response}${responseData ? (" | " + responseData) : ""}`
+  )
 );

I am able to make a pull request for these changes.