Suppress warnings when cache path does not exist. (#608)

* Added an exception clause that catches `FileNotFoundError` and logs a debug message in `SpotifyOAuth.get_cached_token`, `SpotifyPKCE.get_cached_token` and `SpotifyImplicitGrant.get_cached_token`.

* Changed docs for `auth` parameter of `Spotify.init` to `access token` instead of `authorization token`. In issue #599, a user confused the access token with the authorization code.

* Updated CHANGELOG.md

* Removed `FileNotFoundError` because it does not exist in python 2.7 (*sigh*) and replaced it with a call to `os.path.exists`.

* Replaced ` os.path.exists` with `error.errno == errno.ENOENT` to supress errors when the cache file does not exist.

CHANGELOG.md

@@ -9,7 +9,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## Unreleased
 
 ### Changed
+
 - moved os.remove(session_cache_path()) inside try block to avoid TypeError on app.py example file
+- A warning will no longer be emitted when the cache file does not exist at the specified path
+- The docs for the `auth` parameter of `Spotify.init` use the term "access token" instead of "authorization token"  
 
 ## [2.16.1] - 2020-10-24