[nextpnr_fpga_interchange] Fails when encountering unsupported XDC commands, provides warnings for others

[clavin-xlnx]

When parsing Xilinx XDC constraint files, nextpnr_fpga_interchange some commands produce warning messages and continue on while others cause immediate failure. Some of these commands are benign in the context of the XDC file. Since providing port placement constraints is currently required (see #120), a user has to manually filter out the offending XDC commands which is a tedious process. Some of the commands encountered were:

• current_design
• current_instance
• get_ports
• set_false_path
• get_pins
• create_clock
• create_waiver
• create_generated_clock

However, I'm sure there are others. It would be best to generate a warning and move on rather than halt the process completely. A limit on the number of messages (or a summary at the end of XDC parsing) would be useful to not bombard the user with too much repetitive information.