Error messages for datatype validation should be more telling

[xrotwang]

Just raising blank ValueError here

csvw/src/csvw/metadata.py

Lines 322 to 340 in 85d5821

	def validate(self, v):
	if v is None:
	return v
	try:
	l_ = len(v or '')
	if self.length is not None and l_ != self.length:
	raise ValueError()
	if self.minLength is not None and l_ < self.minLength:
	raise ValueError()
	if self.maxLength is not None and l_ > self.maxLength:
	raise ValueError()
	except TypeError:
	pass
	if self.basetype.minmax:
	if self.minimum is not None and v < self.minimum:
	raise ValueError()
	if self.maximum is not None and v > self.maximum:
	raise ValueError()
	return v

isn't very helpful.