Return an error (rather than panic'ing) when a non-serializable user input is provided for `@schema/default`

src: https://github.com/vmware-tanzu/carvel-ytt/pull/490

- today, we generally panic when someone includes a function value (i.e. lambda) in the YAML structure. Ideally, we would _error_ when it’s anticipated bad user input and _panic_ when it is the result of `ytt` coding error. ([example](https://github.com/vmware-tanzu/carvel-ytt/pull/490/files#diff-3cd3d8e2686249d524141b98b510ce20456b45eb99a8a1b082ebbd77602d862dR129))

repro example:
```
#@data/values-schema
---
nothing: "something"
#@schema/default lambda: "hellop"
string: ""
```

`ytt -f .`

Start by reproducing the provided example with `ytt -f .`, then inspect the implementation and tests changed or referenced by pull request 490. The work is done when a non-serializable function value supplied through `@schema/default` reports an error for bad user input instead of panicking, while genuine ytt coding errors can still panic.

Written by the indexing model from the issue text.