buildstats.py: skip collecting unavailable /proc data
Some virtualized environments like Linux-VServer do not have the
entries under /proc that the new system usage sampling expected,
leading to an exception when trying to open the files.
Now the presence of these files is checked once before enabling the
corresponding data collection. When a file is missing, the
corresponding log file is not written either and pybootchart will not
draw the chart that normally displays the data.
Errors while reading or writing of data samples is intentionally still
a fatal error, because that points towards a bigger problem that
should not be ignored.
Reported-by: Andreas Oberritter <obi@opendreambox.org> Signed-off-by: Patrick Ohly <patrick.ohly@intel.com> Signed-off-by: Ross Burton <ross.burton@intel.com>
