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 files changed, 4 insertions, 3 deletions

diff --git a/icebox/icebox_vlog.py b/icebox/icebox_vlog.py
index 21ffb12..9f5b602 100755
--- a/icebox/icebox_vlog.py
+++ b/icebox/icebox_vlog.py
@@ -941,8 +941,9 @@ print("endmodule")
 print()
 
 if failed_drivers_check:
-    print("// Single-driver-check failed for %d nets:" % len(failed_drivers_check))
+    emsg = ["Single-driver-check failed for %d nets:" % len(failed_drivers_check)]
     for net, drivers in failed_drivers_check:
-        print("// %s has %d drivers: %s" % (net, len(drivers), drivers))
-    assert False
+        emsg.append("%s has %d drivers: %s" % (net, len(drivers), drivers))
+    print("//", "\n//".join(emsg))
+    assert False, "\n  ".join(emsg)