Print the correct error when our DynamicCheckerFunctions fail to install

Summary:
The check is inverted here: If we have error messages, we should print those instead
of our default error message. But currently we print the default message when we
actually have a sensible error to print.

Fixes https://bugs.llvm.org/show_bug.cgi?id=38383
Thanks Nat for the patch!

Reviewers: #lldb, JDevlieghere

Reviewed By: JDevlieghere

Subscribers: JDevlieghere, lldb-commits

Differential Revision: https://reviews.llvm.org/D51602

llvm-svn: 341940