Use a wrapper for PyErr_Fetch

This introduces a new class that wraps PyErr_Fetch and PyErr_Restore,
and then changes all the callers in gdb to use it.  This reduces the
amount of explicit reference counting that is done in the Python code.
I also found and fixed a latent bug in gdbpy_print_stack -- it was not
correctly checking some error conditions, nor clearing the exception
when needed.

gdb/ChangeLog
2019-01-03  Tom Tromey  <tom@tromey.com>

	* python/python.c (gdbpy_enter, ~gdbpy_enter): Update.
	(gdbpy_print_stack): Use gdbpy_err_fetch.
	* python/python-internal.h (class gdbpy_err_fetch): New class.
	(class gdbpy_enter) <m_error_type, m_error_value,
	m_error_traceback>: Remove.
	<m_error>: New member.
	(gdbpy_exception_to_string): Don't declare.
	* python/py-varobj.c (py_varobj_iter_next): Use gdbpy_err_fetch.
	* python/py-value.c (convert_value_from_python): Use
	gdbpy_err_fetch.
	* python/py-utils.c (gdbpy_err_fetch::to_string): Rename from
	gdbpy_exception_to_string.
	(gdbpy_handle_exception): Use gdbpy_err_fetch.
	* python/py-prettyprint.c (print_stack_unless_memory_error): Use
	gdbpy_err_fetch.

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