Update type checker to hide undefined function errors if possible

[tsandall]

The type checker currently reports "undefined function" errors if a rule/query refers to a function that itself contains type errors. If an error is introduced into a function that's called in a large number of locations the errors reported by OPA can be extremely noisy and make it difficult to understand the problem (especially since by default OPA will only report up to 10 errors before stopping.)

In cases where "undefined function" errors are due to the referenced function itself containing an error, the type checker can drop/ignore the error. One simple way to implement this would be to simply drop/ignore "undefined function" errors if there is an existing error in the type checking pass.

Example:

package x

p {
	f(1)
}

f(x) {
	max("foo")  # max requires an array
}