base: add SystemErrorCodeToString() function.

Pulls the Windows error string generation out of adb into libbase so
that it can be used by fastboot as well. Also makes a Unix equivalent
that just wraps strerror() so that upcoming fastboot error reporting
code can be platform-independent.

The intent here is just to provide a portable way to report an error to
the user. More general cross-platform error handling is out of scope.

Bug: http://b/26236380
Change-Id: I5a784a844775949562d069bb41dcb0ebd13a32bc