Add an explicit field in bismark-active results for tool exit code.

[woodrow]

• make sure all of our tools actually give non-zero exit codes when they fail (and preferably different codes for different failure modes).
• capture exit code in bismark-$TOOLNAME wrapper script and insert in output xml.

[woodrow]

The measurement tables in the database already have an 'exitstatus' field, so that much is done at least.

   Column   |       Type       |    Modifiers    
------------+------------------+-----------------
...
 exitstatus | integer          | default (-9999)
...

[woodrow]

Started with bismark-mstats.lua (bismark-devel/bismark@74d1792), which now accepts exit status as argument 5, and also properly errors out if it receives something other than numbers. Also added a 'count' attribute to the output XML.

USAGE: ./bismark-mstats.lua param toolname src_ip dst_ip exit_status [direction]
  where a series of real numbers, one per line, is provided to standard input
  to be summarized by ./bismark-mstats.lua

It looks like it's going to take quite a bit of of work to rearrange bismark-measure-active to work properly with this. Currently the bismark-$TOOL output is piped directly into bismark-mstats. We will need to store it in a temporary file so we can capture the exit code (i.e. $?) from bismark-$TOOL. Then we can cat the temporary file into bismark-mstats.lua, while also providing the exit code as an argument to bismark-mstats.lua.