Fail earlier when user try to install future version of R

[rgaiacs]

Closes #1455

[rgaiacs]

What's the situation that leads us here? Can we distinguish between runtime.txt being absent vs having the version and missing the date?

If runtime.txt is absent, the values will be None because of

repo2docker/repo2docker/buildpacks/base.py

Lines 777 to 778 in 1766748

	except FileNotFoundError:
	return self._runtime

From

repo2docker/repo2docker/buildpacks/base.py

Lines 793 to 797 in 1766748

	if len(parts) in (4, 5):
	date = "-".join(parts[-3:])
	if not re.match(r"\d\d\d\d-\d\d-\d\d", date):
	raise ValueError(f"Invalid runtime.txt date: {date}")
	date = datetime.datetime.fromisoformat(date).date()

my understanding is that the data is mandatory but the date might be invalid and raise an exception.

[rgaiacs]

This requires to incorporate #1428.

#1428 was incorporated in 53e8ce1

[manics]

The date is optional

repo2docker/repo2docker/buildpacks/base.py

Lines 763 to 766 in 1766748

	Supported formats:
	name-version
	name-version-yyyy-mm-dd
	name-yyyy-mm-dd