Using output as a source (rather than a dest) throw internal error instead of user error

[timsherwood]

PyRTL throws an "internal error" when an output is used as a source. This should be caught with a proper pyrtl error (earlier in the process if possible).

Traceback (most recent call last):
File "outerr.py", line 9, in
mem_direct[mreg] <<= mout + 1
File "/Users/sherwood/Desktop/work-stuff/GitProjects/PyRTL/pyrtl/wire.py", line 259, in add
return self._two_var_op(other, '+')
File "/Users/sherwood/Desktop/work-stuff/GitProjects/PyRTL/pyrtl/wire.py", line 201, in _two_var_op
working_block().add_net(net)
File "/Users/sherwood/Desktop/work-stuff/GitProjects/PyRTL/pyrtl/core.py", line 297, in add_net
self.sanity_check_net(net)
File "/Users/sherwood/Desktop/work-stuff/GitProjects/PyRTL/pyrtl/core.py", line 569, in sanity_check_net
raise PyrtlInternalError('error, Outputs cannot be arguments for a net')
pyrtl.pyrtlexceptions.PyrtlInternalError: error, Outputs cannot be arguments for a net

Here is a small piece of code that demonstrates the error.

from pyrtl import *

mem_direct = MemBlock(bitwidth=8, addrwidth=8, name='mem_direct')
mout = Output(8, 'mem_direct_output')
mreg = Register(8, 'mreg')

mout <<= mem_direct[mreg]
mreg.next <<= mreg + 1
mem_direct[mreg] <<= mout + 1