Better error reporting/handling for analysis

[brnorris03]

When for some reason the data is not generated with autoperf and user attempts analysis, the error is very cryptic and it's not clear what to do. Handle this case better and give user instructions on what to do next.

norris@compute-0-5:~/temp/autoperf1$ autoperf -y
*** Parsing config files (.autoperf.cfg)... invalid, trying next option...
*** Parsing config files (autoperf.cfg)... done
*** Experiment pi 2015-09-11-14-37-33-149566 is not finished yet, ignore
*** Analyzing pi 2015-09-11-14-41-25-001473 ...
Traceback (most recent call last):
File "/home/norris/autoperf/bin/autoperf", line 243, in
analyze_experiments(options.insnames)
File "/home/norris/autoperf/bin/autoperf", line 234, in analyze_experiments
analyze_an_experiment(exp, insnames)
File "/home/norris/autoperf/bin/autoperf", line 226, in analyze_an_experiment
experiment.analyze()
File "/home/norris/autoperf/lib/python2.7/site-packages/autoperf/experiment.py", line 481, in analyze
self.platform.collect_data()
File "/home/norris/autoperf/lib/python2.7/site-packages/autoperf/platforms/generic.py", line 109, in collect_data
self.tool.collect_data()
File "/home/norris/autoperf/lib/python2.7/site-packages/autoperf/tools/interface.py", line 65, in collect_data
ppk = PPK("%s/data.ppk" % self.experiment.insname, "");
File "/home/norris/autoperf/lib/python2.7/site-packages/autoperf/utils/PPK.py", line 478, in init
f = gzip.open(filename, 'rb')
File "/usr/lib/python2.7/gzip.py", line 34, in open
return GzipFile(filename, mode, compresslevel)
File "/usr/lib/python2.7/gzip.py", line 94, in init
fileobj = self.myfileobj = builtin.open(filename, mode or 'rb')
IOError: [Errno 2] No such file or directory: '2015-09-11-14-41-25-001473/data.ppk'