Add an option not to raise error

[suzuki-shunsuke]

Jsonnet doesn't support catching errors.

• Consider try / catch functionality google/jsonnet#415

So if native functions throw errors, there is no way to handle them.
So it would be great if we add an option not to raise error.
Native Functions can return only one value, so there are two options.

1. Return an object having returned values and an error.

{
  "values": ["support returning multiple values"],
  "error": {}
}

2. Return a value or error.

{
  "type": "io_error",
  "error": true,
  "message": ""
}

To support an option, we have two options.

1. Add new functions

pkg/regexp.MatchString
noerr/regexp.MatchString

2. Add a default argument to function

pkg/regexp.MatchString(pattern, s, opts={})

[suzuki-shunsuke]

Maybe default argument is unavailable.

• Is it possible to define a native function with a default argument? google/go-jsonnet#751

[suzuki-shunsuke]

Rejected.