This patch adds a new convience constructor to the SBError to initialize
it with a string message to avoid having to create the object and call
the `SetErrorString` method afterwards.
This is very handy to report errors from lldb scripted affordances.
Differential Revision: https://reviews.llvm.org/D148401
Signed-off-by: Med Ismail Bennani <medismail.bennani@gmail.com>