Fix handling of Python exceptions during module initialization (#657)

Fixes #656.

Before this commit, the problematic sequence was:

1. `catch (const std::exception &e)` gets a Python exception,
   i.e. `error_already_set`.
2. `PyErr_SetString(PyExc_ImportError, e.what())` sets an `ImportError`.
3. `~error_already_set()` now runs, but `gil_scoped_acquire` fails due
   to an unhandled `ImportError` (which was just set in step 2).

This commit adds a separate catch block for Python exceptions which just
clears the Python error state a little earlier and replaces it with an
`ImportError`, thus making sure that there is only a single Python
exception in flight at a time. (After step 2 in the sequence above,
there were effectively two Python expections set.)

include/pybind11/common.h

@@ -189,6 +189,10 @@ extern "C" {
         }                                                                      \
         try {                                                                  \
             return pybind11_init();                                            \
+        } catch (pybind11::error_already_set &e) {                             \
+            e.clear();                                                         \
+            PyErr_SetString(PyExc_ImportError, e.what());                      \
+            return nullptr;                                                    \
         } catch (const std::exception &e) {                                    \
             PyErr_SetString(PyExc_ImportError, e.what());                      \
             return nullptr;                                                    \
@@ -561,6 +565,9 @@ public:
     /// Give the error back to Python
     void restore() { PyErr_Restore(type, value, trace); type = value = trace = nullptr; }
 
+    /// Clear the held Python error state (the C++ `what()` message remains intact)
+    void clear() { restore(); PyErr_Clear(); }
+
 private:
     PyObject *type, *value, *trace;
 };

include/pybind11/pybind11.h

@@ -1884,8 +1884,7 @@ class gil_scoped_release { };
 error_already_set::~error_already_set() {
     if (value) {
         gil_scoped_acquire gil;
-        PyErr_Restore(type, value, trace);
+        clear();
-        PyErr_Clear();
     }
 }