Always throw errors for bad string conversions

If we're doing things right, the only string conversions are at
initialization time and the results get cached as proper types, so
it shouldn't be too expensive to do an extra if() even in opt mode.

Not as expensive as having a typo in my config file and not noticing
until I see the time steps full of NaNs, certainly...
Merge pull request #155 from grinsfem/string_utils_throw

Always throw errors for bad string conversions
Fix sign error in AdjointStabilization classes
Merge pull request #157 from grinsfem/adj_stab_sign_error

Fix sign error in AdjointStabilization classes
Compatibility with libMesh interior_rate API
More mass_residual, jacobians updates

This adds further corrections for the mass_residual sign convention
change in libMesh and for the "TimeSolvers won't hold your hand and
ensure elem_solution_derivative isn't fractional" change.  The number
of regressions is decreasing but still nonzero...
Adding missing fixed_deriv multipliers