gdbserver: allows agent_mem_read to return an error code

Currently the gdbserver function agent_mem_read ignores any errors
from calling read_inferior_memory.  This means that if there is an
attempt to access invalid memory then this will appear to succeed.

In this patch I update agent_mem_read so that if read_inferior_memory
fails, agent_mem_read will return an error code.

However, none of the callers of agent_mem_read actually check the
return value, so this commit will have no effect on anything.  In the
next commit I will update the users of agent_mem_read to check for the
error code.

I've also updated the header comments on agent_mem_read to better
reflect what the function does, and its possible return values.