lib/message: move xapian call inside try/catch block in _n_m_delete

The call to delete_document can throw exceptions (and can happen in
practice [1]), so catch the exception and extract the error
message. As a side effect, also move the call to _n_m_has_term inside
the try/catch. This should not change anything as that function
already traps any Xapian exceptions.

[1]: id:wwuk039sk2p.fsf@chaotikum.eu