Review error strings

[jonnew]

The error strings in this library often find themselves elevated all the way to the user. It is therefore very important that they are clear. We need to review each of these strings and make sure they describe, succinctly and accurately, what the error code means in an actionable way.

A particularly bad example is the following:

liboni/api/liboni/oni.c

Line 852 in dababf0

return "Attempt to trigger an already triggered operation";

Which should be replaced by something like "Device register access is ongoing and must complete before another is started"

[jonnew]

Combine this with #41 to produce the best error string. This is not specific to register access, so we cant use this verbiage.