fix: strip whitespace and add more detailed error messages

[MarcCon]

Remove all whitespace from the VAT ID and improve error messages to show exactly which part is invalid.

e.g. an invalid number:

parse_vat_id("1DE3456")

ValueError: Invalid country code: 1D

Closes: #79

[barredterra]

I think this function is a good candidate for adding some test cases, since it doesn't depend on the framework: #81

[barredterra]

Reading the issue, my impression is that part of the suggested fix was also something like this (telling the user exactly which field caused the issue):

class VATIDCheck(Document):
	def before_insert(self):
+		try:
			country_code, vat_number = parse_vat_id(self.party_vat_id)
+		except ValueError:
+			frappe.throw(_("The VAT ID in {0} is invalid.").format(
+				_(self.meta.get_label("party_vat_id"))
+			)
		self.party_vat_id = f"{country_code}{vat_number}"