Clarify translations context for error messages

[francoisfreitag]

Since 9923578b435392304c5ac8c56b1f7a8424647d6e, the error message for invalid phone number is annotated with both:

• {example_number} is a national phone number
• {example_number} is an international phone number

The comment does not allow to offer a different translation for each context. Instead, use pgettext to offer context information.