Streamline expression parser error messages.

Currently the expression parser prints a mostly useless generic error before printing the compiler error:

  (lldb) p 1+x)
  error: expression failed to parse:
  error: <user expression 18>:1:3: use of undeclared identifier 'x'
  1+x)
     ^

This is distracting and as far as I can tell only exists to work
around the fact that the first "error: " is unconditionally injected
by CommandReturnObject. The solution is not very elegant, but the
result looks much better.

(Partially addresses rdar://110492710)

Differential Revision: https://reviews.llvm.org/D152590