#### Summary

It would be nice to export a typeguard from Axios' index.d.ts.
This way, consumers trying to do error handling can check in a type-safe way if the error is an AxiosError, or some other kind of error.

It's easy enough to do this as a consumer, but it would be nicer if this was included in the actual Axios library.

test/typescript/axios.ts
axios.interceptors.request.use((config: AxiosRequestConfig) => config);
axios.interceptors.request.use((config: AxiosRequestConfig) => Promise.resolve(config));
const responseInterceptorId: number = axios.interceptors.response.use(
  (response: AxiosResponse) => response,
  (error: any) => Promise.reject(error)
);
axios.interceptors.response.eject(responseInterceptorId);
axios.interceptors.response.use(
  (response: AxiosResponse) => Promise.resolve(response),
  (error: any) => Promise.reject(error)
);
axios.interceptors.response.use((response: AxiosResponse) => response);
axios.interceptors.response.use((response: AxiosResponse) => Promise.resolve(response));
const adapter: AxiosAdapter = (config: AxiosRequestConfig) => {
  const response: AxiosResponse = {
    data: { foo: 'bar' },
    status: 200,
    statusText: 'OK',
    headers: { 'X-FOO': 'bar' },
    config
  };
  return Promise.resolve(response);
};
axios.defaults.adapter = adapter;
const promises = [
  Promise.resolve(1),
  Promise.resolve(2)
];
const promise: Promise<number[]> = axios.all(promises);
const fn1 = (a: number, b: number, c: number) => `${a}-${b}-${c}`;