core, sexp: Ignore malformed sexps when --expect-unexpected is on

Closes GH-50, GH-51.

alectryon/core.py

@@ -126,10 +126,12 @@ def next_sexp(self):
             # https://github.com/ejgallego/coq-serapi/issues/212
             MSG = "SerTop printed an empty line.  Last response: {!r}."
             raise ValueError(MSG.format(self.last_response))
-        self.last_response = response
-        sexp = sx.load(response)
         debug(response, '<< ')
-        return sexp
+        self.last_response = response
+        try:
+            return sx.load(response)
+        except sx.ParseError:
+            return response
 
     def _send(self, sexp):
         s = sx.dump([b'query%d' % self.next_qid, sexp])

alectryon/sexp.py

@@ -73,8 +73,14 @@ def parse(tokens):
             top.append(tok)
     return top[0]
 
+class ParseError(Exception):
+    pass
+
 def load(bs):
-    return parse(tokenize(bs))
+    try:
+        return parse(tokenize(bs))
+    except IndexError:
+        raise ParseError()
 
 def unparse(sexp, buf):
     stack = [sexp]