Duration.create fails silently when given invalid arguments.

[sebright]

Duration.create returns a "zero" value when the arguments are invalid:

opencensus-java/api/src/main/java/io/opencensus/common/Duration.java

Lines 54 to 65 in 773c763

	public static Duration create(long seconds, int nanos) {
	if (seconds < -MAX_SECONDS || seconds > MAX_SECONDS) {
	return ZERO;
	}
	if (nanos < -MAX_NANOS || nanos > MAX_NANOS) {
	return ZERO;
	}
	if ((seconds < 0 && nanos > 0) || (seconds > 0 && nanos < 0)) {
	return ZERO;
	}
	return new AutoValue_Duration(seconds, nanos);
	}

I think this API would be easier to use if it either truncated the values to give a value closer to what the user probably wanted or threw an IllegalArgumentException. Here are two possible ways to change the behavior:

1. If it is important to avoid throwing exceptions, we could truncate the seconds and nanos to their minimum and maximum values. We could throw an IllegalArgumentException if the signs of the seconds and nanos are inconsistent, since it would be unclear what sign the user wanted.

2. We could throw an IllegalArgumentException if the seconds or nanos are out of bounds or have inconsistent sign. This is more consistent with other parts of the API that enforce all class invariants with checkArgument(...).