run: raise a meaningful error message if trace-cmd is not installed
run: raise a meaningful error message if trace-cmd is not installed

The current error message when trace-cmd is not installed is just "File
not found" without even saying which file is not present, which is very
confusing.  Raise a more meaningful error message if we couldn't find
trace-cmd in PATH.

Change-Id: I45c8bc8b1ecd21717bcde854957fdfbbc7d13899

bjackman/trappy,joelagnel/trappy,bjackman/trappy,sinkap/trappy,JaviMerino/trappy,joelagnel/trappy,sinkap/trappy,bjackman/trappy,ARM-software/trappy,sinkap/trappy,sinkap/trappy,joelagnel/trappy,ARM-software/trappy,ARM-software/trappy,JaviMerino/trappy,JaviMerino/trappy,bjackman/trappy,ARM-software/trappy,joelagnel/trappy