transfer read material to own def and handle errors

src/materials.py

@@ -58,11 +58,22 @@ def read_file(filename):
   """\
   Read file as string
   """
+  with open(filename) as data_file:
+    return data_file.read()
+
+
+def read_material(materialname, fullpath=True):
+  """\
+  Read material file as string
+  """
+  filename = os.path.join('data', materialname + '.json')
+  if fullpath:
+    filename = absolute_path(filename)
   try:
-    with open(filename) as data_file:
-      return data_file.read()
+    return read_file(filename)
   except (OSError, IOError) as err:
-    print(str(err))
+    print("Error: Material '%s' not found" % materialname, file=sys.stderr)
+    print(str(err), file=sys.stderr)
     sys.exit(2)
 
 
@@ -171,8 +182,7 @@ def main():
       print_dict(data_dict, False, 2)
 
   elif args.command == 'get':
-    filename = os.path.join('data', args.materialname.lower() + '.json')
-    data = read_file(absolute_path(filename))
+    data = read_material(args.materialname)
     if args.debug:
       print("filename: " + filename)
     if args.const_collection: