useful error message when writing embroidery file fails (fixes #279)

lib/output.py

@@ -84,4 +84,10 @@ def write_embroidery_file(file_path, stitch_plan, svg):
         "full_jump": True,
     }
 
-    pyembroidery.write(pattern, file_path, settings)
+    try:
+        pyembroidery.write(pattern, file_path, settings)
+    except IOError as e:
+        # L10N low-level file error.  %(error)s is (hopefully?) translated by
+        # the user's system automatically.
+        print >> sys.stderr, _("Error writing to %(path)s: %(error)s") % dict(path=file_path, error=e.message)
+        sys.exit(1)

messages.po

@@ -8,7 +8,7 @@ msgid ""
 msgstr ""
 "Project-Id-Version: PROJECT VERSION\n"
 "Report-Msgid-Bugs-To: EMAIL@ADDRESS\n"
-"POT-Creation-Date: 2018-08-24 21:25-0400\n"
+"POT-Creation-Date: 2018-08-24 21:44-0400\n"
 "PO-Revision-Date: YEAR-MO-DA HO:MI+ZONE\n"
 "Last-Translator: FULL NAME <EMAIL@ADDRESS>\n"
 "Language-Team: LANGUAGE <LL@li.org>\n"
@@ -688,6 +688,13 @@ msgstr ""
 msgid "Generate INX files"
 msgstr ""
 
+#. low-level file error.  %(error)s is (hopefully?) translated by
+#. the user's system automatically.
+#: lib/output.py:92
+#, python-format
+msgid "Error writing to %(path)s: %(error)s"
+msgstr ""
+
 #: lib/simulator.py:40
 msgid "Speed up"
 msgstr ""