gdbserver: allow agent expressions to fail with invalid memory access

This commit extends gdbserver to take account of a failed memory
access from agent_mem_read, and to return a new eval_result_type
expr_eval_invalid_memory_access.

I have only updated the agent_mem_read calls related directly to
reading memory, I have not updated any of the calls related to
tracepoint data collection.  This is just because I'm not familiar
with that area of gdb/gdbserver, and I don't want to break anything,
so leaving the existing behaviour untouched seems like the safest
approach.

I've then updated gdb.base/bp-cond-failure.exp to test evaluating the
breakpoints on the target, and have also extended the test so that it
checks for different sizes of memory access.