type-hint: add a more descriptive error when a type hint is unknown

Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>
value-pairs: stricter type hint parsing

Type hints can only consists of alphabetic characters.

Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>
Merge pull request #215 from algernon/f/3.6/type-hints/strict-parsing

Stricter type hint parsing