bigfile/py: Raise MemoryError for ENOMEM errno

Currently we always raise RuntimeError for problems, which is
more-or-less ok for humans, but soon we'll need to distinguish "no
memory" errors from other error conditions in upper layers in code.

Introduce helper function for choosing appropriate exception type for an
error - MemoryError for when errno=ENOMEM and RuntimeError otherwise,
and use it where appropriate.

( Unfortunately Python does not provide such a helper... )

bigfile/_bigfile.c

@@ -106,6 +106,10 @@ typedef struct PyBigFile PyBigFile;
 #define PyType_New(type, typeobj, args) \
     ( (type *)PyObject_CallObject((PyObject *)(typeobj), args) )
 
+/* like PyErr_SetFromErrno(exc), but chooses exception type automatically */
+static void XPyErr_SetFromErrno(void);
+
+
 /************
  *  PyVMA   *
  ************/
@@ -257,7 +261,7 @@ PyFunc(pyfileh_mmap, "mmap(pgoffset, pglen) - map fileh part into memory")
     if (err) {
         Py_DECREF(pyfileh);
         Py_DECREF(pyvma);   // XXX ok wrt delete pyvma->vma ?
-        PyErr_SetFromErrno(PyExc_RuntimeError);
+        XPyErr_SetFromErrno();
         return NULL;
     }
 
@@ -656,7 +660,7 @@ pyfileh_open(PyObject *pyfile0, PyObject *args)
     Py_INCREF(pyfile);
     err = fileh_open(pyfileh, pyfile, ram);
     if (err) {
-        PyErr_SetFromErrno(PyExc_RuntimeError);
+        XPyErr_SetFromErrno();
         Py_DECREF(pyfile);
         Py_DECREF(pyfileh);
         return NULL;
@@ -798,3 +802,17 @@ PyInit__bigfile(void)
 #endif
         _init_bigfile();
 }
+
+
+static void
+XPyErr_SetFromErrno(void)
+{
+    PyObject *exc;
+
+    switch(errno) {
+        case ENOMEM:    exc = PyExc_MemoryError; break;
+        default:        exc = PyExc_RuntimeError;
+    }
+
+    PyErr_SetFromErrno(exc);
+}