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']
```
1 file changed
