lldb: propagate error to user if memory read fails

Previously, an attempt to read an unreadable access reported zeros:

(lldb) memory read -format hex -size 8 0
0x00000000: 0x0000000000000000 0x0000000000000000
0x00000010: 0x0000000000000000 0x0000000000000000
...

Now, if DoReadMemory encounters error then return 0 (bytes read) so we
report the error to the user:

(lldb) memory read -format hex -size 8 0
error: Bad address

LLVM PR: 37190

MFC after: 1 week
Sponsored by: The FreeBSD Foundation