[lldb/API] Add convenience constructor for SBError (NFC)

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>