Crash when solver executable not found

[acfoltzer]

We should handle this situation more gracefully:

 ┏━━━┓━━━┓━┓━┓━┓
 ┃ ━━┓ ╻ ┃ ┃ ┃ ┃
 ┣━━ ┃ ╻ ┃┓ ╻ ┏┛
 ┗━━━┛━┛━┛┗━┛━┛ version 0.2 (5819ac2)

sawscript> prove yices {{ \(x:[32], y:[32], z:[32]) -> x*(y+z) == x*y + x*z }}
saw: Unable to locate executable for Yices
Executable specified: "yices-smt2"

CallStack (from HasCallStack):
  error, called at ./Data/SBV/SMT/SMT.hs:488:23 in sbv-7.7-83dee47a5b386e97a17664820e31896475a2bb5692d636ba96af730da1683cab:Data.SBV.SMT.SMT