[benchtests] Fix compare_strings.py for python2

Python 2 does not have a FileNotFoundError so drop it in favour of
simply printing out the last (and most informative) line of the
exception.

	* benchtests/scripts/compare_strings.py: Import traceback.
	(parse_file): Pretty-print error.

ChangeLog

@@ -1,5 +1,8 @@
 2018-08-02  Siddhesh Poyarekar  <siddhesh@sourceware.org>
 
+	* benchtests/scripts/compare_strings.py: Import traceback.
+	(parse_file): Pretty-print error.
+
 	* NEWS: Mention the change.
 	* elf/dl-tunables.list: Rename tune namespace to cpu.
 	* sysdeps/powerpc/dl-tunables.list: Likewise.

benchtests/scripts/compare_strings.py

@@ -29,6 +29,7 @@ import os
 import json
 import pylab
 import argparse
+import traceback
 
 try:
     import jsonschema as validator
@@ -45,8 +46,8 @@ def parse_file(filename, schema_filename):
                 bench = json.load(benchfile)
                 validator.validate(bench, schema)
         return bench
-    except FileNotFoundError:
-        sys.stderr.write('Invalid input file %s.\n' % filename)
+    except:
+        print(traceback.format_exc(limit=1))
         sys.exit(os.EX_NOINPUT)
 
 def draw_graph(f, v, ifuncs, results):