add CONSTANTS for config file and ConfigError

readformdata.py

@@ -3,7 +3,10 @@
 """Functions for getting data from files.
 
 Constants:
-    COMMENT_SYMBOL = "#"
+    COMMENT_SYMBOL = "#":
+        symbol to indicate a comment line
+    SUBINFO_SEP = "-":
+        symbol to separate subinformation, e.g. for commands
 
 TODO:
     part with FormField.LISTINFOS feels like hardcoded hack.
@@ -14,6 +17,21 @@ import argparse
 from formfield import FormField
 
 COMMENT_SYMBOL = "#"
+SUBINFO_SEP = "-"
+VARIABLE_BEGIN_SEP = "{"
+VARIABLE_END_SEP = "}"
+VARIABLE_PARTS_SEP = "|"
+
+
+class ConfigError(Exception):
+    """There are a lot of mistakes one can make in writing config files.
+
+    This Error indicates that some information is missing or some line is
+    malformed.
+
+    No __init__ supplied because the standard __init__ of Exception does
+    a good job.
+    """
 
 def listFields(fields, newfile):  # NOQA
     """Use pdftk dump_data_fields output to add to a list of data fields.