Improve exception usage in PL/Python

Use the built-in TypeError, not SPIError, for errors having to do with
argument counts or types.  Use SPIError, not simply plpy.Error, for
errors in PLy_spi_execute_plan.  Finally, do not set a Python
exception if PyArg_ParseTuple failed, as it already sets the correct
exception.

Jan Urbański

src/pl/plpython/expected/plpython_unicode_2.out

@@ -39,7 +39,7 @@ SELECT * FROM unicode_test;
 (0 rows)
 
 SELECT unicode_plan1();
-WARNING:  PL/Python: plpy.Error: unrecognized error in PLy_spi_execute_plan
+WARNING:  PL/Python: plpy.SPIError: unrecognized error in PLy_spi_execute_plan
 CONTEXT:  PL/Python function "unicode_plan1"
 ERROR:  PL/Python: plpy.SPIError: PL/Python: could not convert Python Unicode object to PostgreSQL server encoding
 DETAIL:  UnicodeError: ASCII encoding error: ordinal not in range(128)

src/pl/plpython/expected/plpython_unicode_3.out

@@ -39,7 +39,7 @@ SELECT * FROM unicode_test;
 (0 rows)
 
 SELECT unicode_plan1();
-WARNING:  PL/Python: plpy.Error: unrecognized error in PLy_spi_execute_plan
+WARNING:  PL/Python: plpy.SPIError: unrecognized error in PLy_spi_execute_plan
 CONTEXT:  PL/Python function "unicode_plan1"
 ERROR:  PL/Python: plpy.SPIError: PL/Python: could not convert Python Unicode object to PostgreSQL server encoding
 DETAIL:  UnicodeEncodeError: 'ascii' codec can't encode character u'\x80' in position 0: ordinal not in range(128)

src/pl/plpython/plpython.c

@@ -2820,15 +2820,11 @@ PLy_spi_prepare(PyObject *self, PyObject *args)
 	int			nargs;
 
 	if (!PyArg_ParseTuple(args, "s|O", &query, &list))
-	{
-		PLy_exception_set(PLy_exc_spi_error,
-						  "invalid arguments for plpy.prepare");
 		return NULL;
-	}
 
 	if (list && (!PySequence_Check(list)))
 	{
-		PLy_exception_set(PLy_exc_spi_error,
+		PLy_exception_set(PyExc_TypeError,
 					   "second argument of plpy.prepare must be a sequence");
 		return NULL;
 	}
@@ -2984,7 +2980,7 @@ PLy_spi_execute_plan(PyObject *ob, PyObject *list, long limit)
 	{
 		if (!PySequence_Check(list) || PyString_Check(list) || PyUnicode_Check(list))
 		{
-			PLy_exception_set(PLy_exc_spi_error, "plpy.execute takes a sequence as its second argument");
+			PLy_exception_set(PyExc_TypeError, "plpy.execute takes a sequence as its second argument");
 			return NULL;
 		}
 		nargs = PySequence_Length(list);
@@ -3002,7 +2998,7 @@ PLy_spi_execute_plan(PyObject *ob, PyObject *list, long limit)
 		if (!so)
 			PLy_elog(ERROR, "could not execute plan");
 		sv = PyString_AsString(so);
-		PLy_exception_set_plural(PLy_exc_spi_error,
+		PLy_exception_set_plural(PyExc_TypeError,
 							  "Expected sequence of %d argument, got %d: %s",
 							 "Expected sequence of %d arguments, got %d: %s",
 								 plan->nargs,
@@ -3089,7 +3085,7 @@ PLy_spi_execute_plan(PyObject *ob, PyObject *list, long limit)
 		}
 
 		if (!PyErr_Occurred())
-			PLy_exception_set(PLy_exc_error,
+			PLy_exception_set(PLy_exc_spi_error,
 							  "unrecognized error in PLy_spi_execute_plan");
 		PLy_elog(WARNING, NULL);
 		PLy_spi_exception_set(edata);