[Flang] Exit gracefully with a useful message when we fail to lookup a target

[Flang] Exit gracefully with a useful message when we fail to lookup a target

Without this patch we were asserting with a generic message `Failed to
create Target`, but we already have a detailed error message stored in
the variable `error` after calling `lookupTarget()` but this error was not
getting used/printed.

With this patch we will emit a message with more details instead of a
stack dump with a generic message.

Differential Revision: https://reviews.llvm.org/D146333

Change-Id: I7ddee917cf921a2133ca3e6b35791b2142f770a2