Fix 3GPP SMS send failure for 7-bit national language tables.

Fix a NullPointerException when sending a single-part SMS containing
characters in one of the enabled national language tables.

Also added a few log messages for several error cases to help with
debugging any future problems in the SMS dispatcher.

Bug: 5553544
Change-Id: I61c1cbe297b2e222027f0db7c833df6a03c2974a