py: Add mp_errno_to_str() and use it to provide nicer OSError msgs.

If an OSError is raised with an integer argument, and that integer
corresponds to an errno, then the string for the errno is used as the
argument to the exception, instead of the integer.  Only works if
the uerrno module is enabled.

py/moduerrno.c

@@ -89,4 +89,13 @@ const mp_obj_module_t mp_module_uerrno = {
     .globals = (mp_obj_dict_t*)&mp_module_uerrno_globals,
 };
 
+mp_obj_t mp_errno_to_str(mp_obj_t errno_val) {
+    mp_map_elem_t *elem = mp_map_lookup((mp_map_t*)&errorcode_dict.map, errno_val, MP_MAP_LOOKUP);
+    if (elem == NULL) {
+        return errno_val;
+    } else {
+        return elem->value;
+    }
+}
+
 #endif //MICROPY_PY_UERRNO

py/mperrno.h

@@ -134,4 +134,10 @@
 
 #endif
 
+#if MICROPY_PY_UERRNO
+mp_obj_t mp_errno_to_str(mp_obj_t errno_val);
+#else
+static inline mp_obj_t mp_errno_to_str(mp_obj_t errno_val) { return errno_val; }
+#endif
+
 #endif // __MICROPY_INCLUDED_PY_MPERRNO_H__

py/objexcept.c

@@ -36,6 +36,7 @@
 #include "py/objtype.h"
 #include "py/runtime.h"
 #include "py/gc.h"
+#include "py/mperrno.h"
 
 // Instance of MemoryError exception - needed by mp_malloc_fail
 const mp_obj_exception_t mp_const_MemoryError_obj = {{&mp_type_MemoryError}, 0, 0, NULL, (mp_obj_tuple_t*)&mp_const_empty_tuple_obj};
@@ -288,6 +289,10 @@ mp_obj_t mp_obj_new_exception(const mp_obj_type_t *exc_type) {
 
 // "Optimized" version for common(?) case of having 1 exception arg
 mp_obj_t mp_obj_new_exception_arg1(const mp_obj_type_t *exc_type, mp_obj_t arg) {
+    // try to provide a nice string instead of numeric value for errno's
+    if (exc_type == &mp_type_OSError && MP_OBJ_IS_SMALL_INT(arg)) {
+        arg = mp_errno_to_str(arg);
+    }
     return mp_obj_new_exception_args(exc_type, 1, &arg);
 }