Improve error handling for invalid credentials and environment variable

[adriacidre]

I encountered an issue when initializing the self-sdk library with invalid credentials. When this happens, a 401 error is thrown, and the error message is not very informative for the end-user.

The error message looks like this:

[2023-03-24T09:00:54.000Z] WARN AxiosError: Request failed with status code 401
    at settle (/Users/adriancidrejugo/tmp/self-ts-sdk/_examples/authentication/node_modules/axios/dist/node/axios.cjs:1900:12)
    at IncomingMessage.handleStreamEnd (/Users/adriancidrejugo/tmp/self-ts-sdk/_examples/authentication/node_modules/axios/dist/node/axios.cjs:2952:11)
    at IncomingMessage.emit (events.js:326:22)
    at endReadableNT (_stream_readable.js:1241:12)
    at processTicksAndRejections (internal/process/task_queues.js:84:21) {
  code: 'ERR_BAD_REQUEST',
  config: [Object],
  request: [ClientRequest],
  response: [Object]
}
(node:45046) UnhandledPromiseRejectionWarning: Error: internal error
    at new IdentityService (/Users/adriancidrejugo/tmp/self-ts-sdk/_examples/authentication/node_modules/self-sdk/dist/self-sdk.umd.js:967:32)
    at Function.<anonymous> (/Users/adriancidrejugo/tmp/self-ts-sdk/_examples/authentication/node_modules/self-sdk/dist/self-sdk.umd.js:8139:51)
    at step (/Users/adriancidrejugo/tmp/self-ts-sdk/_examples/authentication/node_modules/self-sdk/dist/self-sdk.umd.js:107:27)
    at Object.next (/Users/adriancidrejugo/tmp/self-ts-sdk/_examples/authentication/node_modules/self-sdk/dist/self-sdk.umd.js:88:57)
    at fulfilled (/Users/adriancidrejugo/tmp/self-ts-sdk/_examples/authentication/node_modules/self-sdk/dist/self-sdk.umd.js:78:62)
    at processTicksAndRejections (internal/process/task_queues.js:97:5)
(node:45046) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 2)
(node:45046) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

To improve the user experience and provide better guidance for troubleshooting, I suggest improving the error handling in the library when initializing with invalid credentials or when the SELF_ENV environment variable is set incorrectly.