Fix error for evaluate APIs

[ankur22]

What?

This fixes the error that is returned when a user works with the evaluate APIs when a navigation isn't occurring:

  const page = browser.newPage();

  const blah = page.evaluate("1+1");
  console.log(blah);
  
  page.close();

This used to return GoError: evaluating JS: execution context changed; most likely because of a navigation, but now it returns given expression does not evaluate to a function.

Why?

The new error better represents what the user can do to fix the problem instead of being left confused when no navigation has occurred. In the example above and with the new error, they should be able to work out that evaluate only works with callable functions.

Checklist

• I have performed a self-review of my code
• I have added tests for my changes
• I have commented on my code, particularly in hard-to-understand areas

Related PR(s)/Issue(s)

/~https://github.com/grafana/k6-cloud/issues/2066

[inancgumus]

LGTM 👍

Some suggestions.

There is also a linter error:

TestPageEvaluateMapping should call t.Parallel on the top level as well as its subtests (parallel)