Display better error message for missing credentials (#27)

```
irb(main):002:0> OmniAuth::Strategies::NitroId.new({}).client
Traceback (most recent call last):
        5: from bin/console:8:in `<main>'
        4: from (irb):1
        3: from (irb):2:in `rescue in irb_binding'
        2: from /Users/greersa/Workspace/omniauth-nitro-id/lib/omniauth/strategies/base_strategy.rb:22:in `client'
        1: from /Users/greersa/Workspace/omniauth-nitro-id/lib/omniauth/strategies/base_strategy.rb:25:in `rescue in client'
OmniAuth::Strategies::BaseStrategy::ClientCredentialsError (NitroId client credentials not found. Please check your environment.)
```

lib/omniauth/strategies/base_strategy.rb

@@ -7,6 +7,7 @@ module OmniAuth
   module Strategies
     class BaseStrategy < OmniAuth::Strategies::OpenIDConnect
       class APIError < StandardError; end
+      class ClientCredentialsError < StandardError; end
 
       def public_key
         @public_key ||= if options.discovery
@@ -18,6 +19,13 @@ def public_key
                         end
       end
 
+      def client
+        super
+      rescue AttrRequired::AttrMissing
+        raise ClientCredentialsError,
+              "#{options[:name].camelize} client credentials not found. Please check your environment."
+      end
+
       def self.decode_logout_token(token)
         jwks = fetch_jwks
         JSON::JWT.decode(token, jwks)