"datasette inspect" outputs invalid JSON if an error is logged

See https://github.com/simonw/register-of-members-interests/issues/6:
```
% datasette inspect regmem.db                       
ERROR: conn=<sqlite3.Connection object at 0x10d2a9c60>, sql = 'select count(*) from [items_fts]', params = None: SQL logic error
{
  "regmem": {
    "hash": "6fde27e3dea80d6b65f2ac7f89cd8448980fee8c91b505ba29c311ba0393317f",
    "size": 936198144,
```