Clarifies bailout strategy for avoiding known errors

Bailout Strategy
1. A _missing_ filter.filterValue will results in an
   error when passed to mozContacts.find, bailing out
   early avoids the additional overhead of making a
   request that we know will fail.
2. Missing navigator.mozContacts API or empty but
   present filter.filterValue make the app useless,
   so prevent unpredictable state by making the
   result a no-op.

Signed-off-by: Rick Waldron <waldron.rick@gmail.com>
Failing tests: multi-term lookups where predominate term is the first in the terms list (mixed letter case)

Signed-off-by: Rick Waldron <waldron.rick@gmail.com>
Fixes: multi-term lookups where predominate term is the first in the terms list (mixed letter case)

Signed-off-by: Rick Waldron <waldron.rick@gmail.com>