`FrozenShock` allows shock velocities below the speed of sound

[corykinney]

Expected behavior:
FrozenShock should raise an exception for a u1 corresponding to a Mach number below 1.

[corykinney]

Although FrozenShock generally crashes for a subsonic u1, an explicity check will prevent any chance of successfully arriving at an incorrect solution. Additionally, a more helpful error message is needed instead of a confusing linear algebra error that doesn't clearly indicate the actual issue.