Add error message to socket errors (instead of just errno).

Changes `socketError(..)` output from:

> Failed to connect to server, rc: -1, errno: 113

to:

> Failed to connect to server, rc: -1. Error: 'No route to host' (errno: 113)

which should give some more information to the user.

Fix #29.
Merge pull request #30 from gavanderhoorn/issue29_errno_and_text

Fix issue #29: Log messages that display errno should display the associated error text instead.

I pulled in this change even though it may not be 100% cross platform because predicting how/what standard linux functions each platform will implement is difficult.  Cross platform issues can only be addressed as we encounter them.  In addition, the cross platform support was primarily maintained for the motoman dx100 package, which no longer uses the simple_message library.