Error message: Can we improve error message on API errors due to shipyard parsing

[kethahel99]

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Environment
Any keptn installation

• Client OS (e.g., Linux, macOS, Windows): Windows
• Keptn Version (keptn version): 0.11.4
• Kubernetes Cloud Provider (e.g., GKE, AKS): AKS
• Kubernetes version (kubectl version):1.20.9

N/A

Affected Component

• Approval
• Bridge
• CLI
• Datastore
• Docs
• Distributor / Event Broker
• Helm
• Istio
• Jmeter
• Kubernetes Integration
• Openshift Integration
• REST API
• Webhook

Describe the bug
If you shipyard.yaml is not properly formatted .e.g indentiation, then you will receive and API error "sequence tasks not iterable" message . As a result, you might have later (e.g. webhook or job-executor service) when trying to configure event subscriptions in the bridge

To Reproduce

Steps to reproduce the behavior:

1. Change you shipyard.yaml file and decrease the indentiation of a task or tasks under one or more sequences e.g:

apiVersion: "spec.keptn.sh/0.2.2"
kind: "Shipyard"
metadata:
  name: "shipyard-quality-gates"
spec:
  stages:
  - name: "quality-gate"
    sequences:
      - name: evaluation
        tasks:
        - name: monaco
        - name: evaluation
      - name: simple_evaluation
        tasks:
      - name: evaluation
        triggeredAfter: "6m"
        properties:
          timeframe: "6m"      

Note, that the evaluation task in the simple_evaluation sequence should have indentation for this to work properly e.g.

apiVersion: "spec.keptn.sh/0.2.2"
kind: "Shipyard"
metadata:
  name: "shipyard-quality-gates"
spec:
  stages:
  - name: "quality-gate"
    sequences:
      - name: evaluation
        tasks:
        - name: monaco
        - name: evaluation
      - name: simple_evaluation
        tasks:
        - name: evaluation
          triggeredAfter: "6m"
          properties:
            timeframe: "6m"      

2. Go to Uniform
3. Click on e.g. webhook-service
4. Click on Add subscription
5. The bridge UI will show a loading screen and the console will eventually report an HTTP 500 error
6. See error
   

Expected behavior
Perhaps a more clear error mesasge like:

"The API cannot iterate though the tasks in your sequence : {sequence name}. Please verify that your shipyard.yaml is correct. Ensure to check indentation and format."

Current behavior
API error given is "sequence tasks not iterable" and error is not handled with clear message on the UI

Screenshots

Additional context
N/A