[issue1029] better error message for missing "(define "

In the translation algorithm, the assertion now contains some details
about what went wrong in when a domain or a problem does not start
with "(define ".

CHANGES.md

@@ -11,6 +11,9 @@ after the corresponding tracker issues.
 
 ## Changes since the last release
 
+- Better error message when domain does not start with `(define`.
+  <http://issues.fast-downward.org/issue1029>
+
 - Add debugging methods to LP solver interface.
   <http://issues.fast-downward.org/issue960>
   You can now assign names to LP variables and constraints for easier

src/translate/pddl_parser/parsing_functions.py

@@ -317,7 +317,10 @@ def parse_domain_pddl(domain_pddl):
     iterator = iter(domain_pddl)
 
     define_tag = next(iterator)
-    assert define_tag == "define"
+    assert define_tag == "define", (
+            "domain specification must begin with \"(define \", "
+            "found \"(%s \" instead." % define_tag
+        )
     domain_line = next(iterator)
     assert domain_line[0] == "domain" and len(domain_line) == 2
     yield domain_line[1]