assert should manifest the error message

[szeiger]

assert is specified as desugaring to error. We handle error correctly in Sjsonnet: The error message is manifested as a string. But in the case of assert we cast it to a string instead, so it does not work for other types.

Compiling error {a: "b"}:

sjsonnet.Error: {"a": "b"}
    at [Error].(foo:1:1)

RUNTIME ERROR: {"a": "b"}
	foo:1:1-15

Compiling assert 1 == 2 : { a: "b"}; 1:

sjsonnet.Error: Expected string, found object
    at [AssertExpr].(foo:1:1)

RUNTIME ERROR: {"a": "b"}
	foo:1:1-29