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

[pivotaljohn]

src: #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)

repro example:

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

ytt -f .