Display better error message for missing credentials

[stephenagreer]

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.)

[web-kat]

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