An SBValue whose underlying ValueObject has no valid value, but does

hold an error should:

(a) return false for IsValid, since that's the current behavior and is
    a convenient way to check "should I get the value for this".
(b) preserve the error when an SBValue is made from it, and print the
    error in the ValueObjectPrinter.

Make that happen.

Differential Revision: https://reviews.llvm.org/D144664