Introduce fatal error for GB with FE

Old code silently computed zeroes for Coulomb(SR) when using
Generalized Born and the free energy kernel, because of the
inappropriate use of a default branch of a switch statement.

Fixed the default branch for icoul and ivdw switch statements to issue
gmx_incons (similar to usage in the Verlet kernel code). Now we invoke
explicit code paths for all legal values of icoul and ivdw. Added
gmx_fatal for un-implemented GB case (similar to the gmx_fatal for
Buckingham with this free energy kernel).

Partial fix for #1237

Change-Id: I30684aed9fba9beac9b1b87eb99e2cbc7e7e374d