Raise custom error if missing configuration

[lucascarrias]

This is an improvement suggestion.

How?

• #push method is called
• config.json_key_io is nil
• Some Google's env var is nil

Reference: https://github.com/miyataka/fcmpush#usage

What?

The following error is raised:

NoMethodError:
 undefined method `gsub' for nil:NilClass

Suggestion

Rescue the NoMethodError and raises a custom error with information regarding the missing configuration

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Thanks for your time and for the good work.

[miyataka]

@LucasCarrias Thanks for your suggestion. That's sounds good. It helps that fcmpush newbie.