improve parser error messages

Author: braised-babbage

source/openpulse/openpulse/parser.py

@@ -87,7 +87,14 @@ def parse_openpulse(
     try:
         tree = parser.calibrationBlock()
     except (RecognitionException, ParseCancellationException) as exc:
-        raise OpenPulseParsingError() from exc
+        msg = ''
+        # With BailErrorStrategy, we should be able to recover and report
+        # information about the offending token.
+        if isinstance(exc, ParseCancellationException) and exc.args:
+            tok = getattr(exc.args[0], 'offendingToken', None)
+            if tok is not None:
+                msg = f"Unexpected token '{tok.text}' at line {tok.line}, column {tok.start}."
+        raise OpenPulseParsingError(msg) from exc
     result = (
         OpenPulseNodeVisitor(in_defcal).visitCalibrationBlock(tree)
         if tree.children

source/openpulse/tests/test_openpulse_parser.py

@@ -383,7 +383,7 @@ def test_permissive_parsing(capsys):
     captured = capsys.readouterr()
     assert captured.err.strip() == "line 2:9 no viable alternative at input 'int;'"
 
-    with pytest.raises(OpenPulseParsingError):
+    with pytest.raises(OpenPulseParsingError, match=r"Unexpected token ';' at line 2, column 10."):
         # This is stricter -- we fail as soon as ANTLR sees a problem
         parse(p)
     captured = capsys.readouterr()