Add function for getting error strings

[Nicholas Humfrey]

Please add a function to the client library, to get an error message string for an error number.

Something like:
const char* mosquitto_strerror(int mosq_errno)

It may also be preferable to put the error numbers into a enum, so that when using a switch statement, the compiler can check that you have implemented all the values.

[Roger Light (roger.light)]

Sounds reasonable. Does using enums have any implication for ABI stability? I had a feeling #defines were better but I can't find anything that says enums will break binary compatibility if you add to them.

[Roger Light (roger.light)]

The function now exists but may need some string changes:

https://bitbucket.org/oojah/mosquitto/changeset/cca382120ddb

I'm still thinking about enums.

[Nicholas Humfrey (njh-aelius)]

It is pretty safe to use enums to define values.

Googling seems to imply that there are some potential ABI issues with using enums for parameters to functions and return types - their size isn't guaranteed on all platforms.

However it should be safe to cast them to/from an integer.