Set error message if ValueObjectRegister fails to write back to register

SetValueFromCString and SetData methods return false if register can't
be written but they don't set a error message. It sometimes confuses
callers of these methods because they try to get the error message in case of
failure but Status::AsCString returns nullptr.

For example, lldb-vscode crashes due to this bug if some register can't
be written. It invokes SBError::GetCString in case of error and doesn't
check whether the result is nullptr (see request_setVariable implementation in
lldb-vscode.cpp for more info).

Reviewed By: labath, clayborg

Differential Revision: https://reviews.llvm.org/D120319