fio2gnuplot: Print line if int() conversion fails

When data aren't properly formated the int() conversion could fails but
doesn't print the impacted line which is pretty difficult to estimate
where the bug is.

This patch simply reports the line that cause the trouble.
diff --git a/tools/plot/fio2gnuplot.py b/tools/plot/fio2gnuplot.py
index 4be011d..c057991 100755
--- a/tools/plot/fio2gnuplot.py
+++ b/tools/plot/fio2gnuplot.py
@@ -179,7 +179,12 @@
 		for line in current_line:
 			time, perf, x, block_size = line
 			if (blk_size == 0):
-				blk_size=int(block_size)
+				try:
+					blk_size=int(block_size)
+				except:
+					print "Error while reading the following line :"
+					print line
+					sys.exit(1);
 
 			# We ignore the first 500msec as it doesn't seems to be part of the real benchmark
 			# Time < 500 usually reports BW=0 breaking the min computing