Use gdbpy_convert_exception in a few more spots

Use gdbpy_convert_exception in a few more spots

I noticed a few places were converting a gdb exception to a Python
exception "by hand".  It's better to use the existing
gdbpy_convert_exception helper function, as this handles memory errors
correctly, and in the future may be enhanced in other ways.

gdb/ChangeLog
2018-12-27  Tom Tromey  <tom@tromey.com>

	* python/py-value.c (convert_value_from_python): Use
	gdbpy_convert_exception.
	* python/py-param.c (parmpy_init): Use gdbpy_convert_exception.
	* python/py-cmd.c (cmdpy_init): Use gdbpy_convert_exception.
	* python/py-breakpoint.c (bppy_init): Use
	gdbpy_convert_exception.