Make symlink error message more helpful

[cari-lynn]

Describe the problem/challenge you have
Using symlinks like in Kubernetes to mount a configmap or secret into a container causes a error unless using --allow-symlink-destination. This error message is confusing as to why symlinks aren't allowed by default.

The current error:

$ ytt -f /opt/xxx/config/workshop.yaml
ytt: Error: Checking symlink file '/opt/xxx/config/workshop.yaml':
  Expected symlink file '/opt/xxx/config/workshop.yaml' -> '/opt/xxx/config/..2022_04_09_00_13_38.3873050756/workshop.yaml' to be allowed, but was not

Describe the solution you'd like
A more clear error that describes how to solve the problem:

$ ytt -f /opt/xxx/config/workshop.yaml
ytt: Error: Checking symlink file '/opt/xxx/config/workshop.yaml':
  Expected symlink file '/opt/xxx/config/workshop.yaml' -> '/opt/xxx/config/..2022_04_09_00_13_38.3873050756/workshop.yaml' to be allowed, but was not. Symlinks are disallowed as a security feature, use `--allow-symlink-destination` flags to override

Anything else you would like to add:
Originating issue: #643

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[cppforlife]

this is a good example for using hint:

$ ytt -f /opt/xxx/config/workshop.yaml
ytt: Error: Checking symlink file '/opt/xxx/config/workshop.yaml':
  Expected symlink file '/opt/xxx/config/workshop.yaml' -> '/opt/xxx/config/..2022_04_09_00_13_38.3873050756/workshop.yaml' to be allowed, but was not (hint: symlinks are disallowed as a security feature, use '--allow-symlink-destination' flags to override)

[vicmarbev]

I would like to work on it!

[cari-lynn]

@vicmarbev That's great to hear! We will keep an eye out for a PR from you. Let us know if you have any questions here or in the Kubernetes Slack in #carvel channel. :)

[cppforlife]

merged!