Better error handling with templates

[vickicello]

The Issue

Currently, if you try to template an individual secret object by parsing it out of a JSON file returned by all-secrets kind, and that individual secret doesn't exist in the JSON, AKVA will panic and fail. For example:

    resources:
      - kind: all-secrets
        vaultBaseURL: https://my-testing.vault.azure.net/
    sinks:
      - path: secrets.json
        template: "{{ index .Secrets | toValues | toJson }}"
      - path: testcert.pem
        template: '{{ index .Secrets "testcert" | fullChain }}' # testcert.pem does not exist in secrets.json

...

AKVA logs show the following:

{"_timestamp":"2023-05-25T13:16:21-04:00","level":"fatal","msg":"Caught Panic In Worker: Error executing template: {{ in
dex .Secrets \"testcert\" | fullChain }} Error: template: template:1:43: executing \"template\" at 'ful
lChain': error calling fullChain: runtime error: invalid memory address or nil pointer dereference"}

Proposed Solution

We should update this code to loop through the resource sinks twice. The first pass will render all of the templates, and if a template fails, AKVA should bail out (but not crash). If all templated items render correctly in the first loop, then AKVA should write them all in the second loop.