Fix crash on not handled Oauth2.Error

[lukaszsamson]

** (CaseClauseError) no case clause matching: {:error, %OAuth2.Error{reason: :timeout}}

See spec for https://hexdocs.pm/oauth2/OAuth2.Client.html#get_token/4

get_token(client, params \\ [], headers \\ [], opts \\ [])View Source
get_token(t(), params(), headers(), Keyword.t()) ::
  {:ok, OAuth2.Client.t()}
  | {:error, OAuth2.Response.t()}
  | {:error, OAuth2.Error.t()}