Add missing assert.error method

[erights]

assert.error is an obviously missing method. Exactly like assert.fail for making an error with associated details. But unlike assert.fail it just returns the error rather than throwing it.

Currently the only way to get this effect from outside the assert module is

let err;
try {
  assert.fail(d`...`);
} catch (e) {
  err = e;
}

which would be just horrible.

Motivation:
While debugging a change to Agoric/agoric-sdk#1905 I'm seeing places where we're not logging info we should be due to the lack of this method.

[kriskowal]

Maker names will be a tough sell if this goes up for standardization. assert.Error would be the easiest fit if it can be a constructor, assert.error is not so bad.

[erights]

Maker names will be a tough sell if this goes up for standardization. assert.Error would be the easiest fit if it can be a constructor, assert.error is not so bad.

Done