icebox_vlog: Save error message to file and print it.

Previously if you were doing;

`icebox_vlog example.asc > example_bit.v` you would just get;

```
Traceback (most recent call last):
  File "icebox_vlog.py", line 947, in <module>
    assert False
AssertionError
```

Now you get;
```
Traceback (most recent call last):
  File "icebox_vlog.py", line 948, in <module>
    assert False, "\n  ".join(emsg)
AssertionError: Single-driver-check failed for 2 nets:
  n10 has 0 drivers: []
  n15 has 2 drivers: ['clk', 'clk2']
```

0 files changed, 0 insertions, 0 deletions