Check returncode of gnuplot to detect errors

Do not try to upload image, if gnuplot run fails

Issue #2 (https://git.golderweb.de/wiki/jogobot-euroexchange/issues/2)

euroexchange/euroexchange.py

@@ -247,6 +247,7 @@ class EuroExchangeBot( pywikibot.bot.BaseBot ):
 
         # Check if update is necessary
         if self.image_update_needed():
+            try:
                 self.call_gnuplot( job )
 
                 if self.file_changed():
@@ -255,6 +256,10 @@ class EuroExchangeBot( pywikibot.bot.BaseBot ):
                     jogobot.output( "No upload needed for Job {}.".format(
                     self.current_job.image) )
 
+            except subprocess.CalledProcessError as e:
+                jogobot.output( "Subprocess terminated with exit code {}!".
+                format( e.returncode), "ERROR" )
+
         # Nothing to do
         else:
             jogobot.output( "No update needed for Job {}".format(
@@ -299,7 +304,7 @@ class EuroExchangeBot( pywikibot.bot.BaseBot ):
         plt_env["INFILE"] = type(self).csv_file
         plt_env["OUTFILE"] = job.image
 
-        subprocess.call( cmd, cwd=self.wdir, env=plt_env )
+        subprocess.check_call( cmd, cwd=self.wdir, env=plt_env )
 
     def file_changed( self ):
         """