ayer-create-trunk: fix parameter checking

Simple bug allowed to run ayer-create-trunk with absent or empty second
parameter.

Change-Id: I9e613006c6033a96caa871b72e0f1c0de287bf8d
Check that functions.sh was successfully sourced

Since 'set -e' is performed in functions.sh, it is sourced without
errexit option set, and script execution might contiune on failure.
This might happen if ayer installation is broken.

Change-Id: Ic65bb1cd384479bb5de8854698993b5fcec54c51