Note: This report was initially about throwing an error if non existing file is referenced in a variable. It was updated to cover all failed variable resolution cases

Ideally we should fail unconditionally whenever resolution of a variable from a source crashes.
Note that fallbacks should be applied if no value was resolved (null or undefined), but not when resolution crashes.

Cases it should cover (by variables scope):

• file: Pointing non existing file
• cf, s3, ssm: Inability to retrieve the data (network or authentication error)
• env: Not set environment variable

Proposed implementation

1. Introduce a configuration option: unresolvedVariablesNotificationMode with two possible options 'warn' (default) and 'error' (In v2 we will make 'error' a default)
2. Throw on unresolved variables if unresolvedVariablesNotificationMode is set to 'error'
3. Introduce a following deprecation log if there are unresolved variables in a service, and service configuration doesn't contain unresolvedVariablesNotificationMode:

Unresolved variable references will be communicated with a thrown error, starting from next major 

See https://github.com/serverless/serverless/blob/master/CONTRIBUTING.md#configuring-deprecations for info on how to configure deprecations