[Validation] Fix bug that caused unhelpful validation errors in case DevSettings is null, DevSettings/InstanceTypesData is not a valid JSON or it is missing required fields.

[gmarciani]

Description of changes

Fix bugs that caused unhelpful validation error messages in the following cases:

1. DevSettings is null.
2. DevSettings/InstanceTypesData is not a valid JSON.
3. DevSettings/InstanceTypesData is missing required fields.

Before this change the validators where returning cryptic and not actionable error messages such as 'NoneType' object has no attribute 'get'.

With this change we moved to more clear an acitonable error messages (See UX description below).

User experience after this change

When DevSettings is null:

{
  "configurationValidationErrors": [
    {
      "level": "ERROR",
      "type": "ConfigSchemaValidator",
      "message": "[('DevSettings', ['Field may not be null.'])]"
    }
  ],
  "message": "Invalid cluster configuration."
}

When DevSetting/InstanceTypesData is not a valid JSON:

{
  "message": "Invalid cluster configuration: DevSettings/InstanceTypesData is not a valid JSON."
}

When DevSettings/InstanceTypesData is missing the required field NetworkInfo/NetworkCards

{
  "configurationValidationErrors": [
    {
      "level": "ERROR",
      "type": "MultiNetworkInterfacesInstancesValidator",
      "message": "Could not determine network cards for instance type hpc5a.48xlarge"
    },
    {
      "level": "ERROR",
      "type": "ComputeResourceLaunchTemplateValidator",
      "message": "Unable to validate configuration parameters for queue q1. Could not determine network cards for instance type hpc5a.48xlarge"
    },
    {
      "level": "ERROR",
      "type": "InstancesNetworkingValidator",
      "message": "Could not determine network cards for instance type hpc5a.48xlarge"
    }
  ],
  "message": "Invalid cluster configuration."
}

User Experience before this change

When DevSettings is null:

{
  "message": "Invalid cluster configuration: 'NoneType' object has no attribute 'get'"
}

When DevSettings/InstanceTypesData is not a valid JSON:

{
  "message": "Invalid cluster configuration: Expecting ':' delimiter: line 1 column 16 (char 15)"
}

When DevSettings/InstanceTypesData is missing the requiredf field NetworkInfo/NetworkCards

{
  "configurationValidationErrors": [
    {
      "level": "ERROR",
      "type": "MultiNetworkInterfacesInstancesValidator",
      "message": "object of type 'NoneType' has no len()"
    },
    {
      "level": "ERROR",
      "type": "ComputeResourceLaunchTemplateValidator",
      "message": "Unable to validate configuration parameters for queue q1. object of type 'NoneType' has no len()"
    },
    {
      "level": "ERROR",
      "type": "InstancesNetworkingValidator",
      "message": "object of type 'NoneType' has no len()"
    }
  ],
  "message": "Invalid cluster configuration."
}

Tests

• Manual validation of the UX described above.
• Unit tests (existing and new ones)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.