Investigate why many bridge measurements fail

I don't know whether it's because at the beginning the scanner is still busy obtaining and caching descriptors, or just bad luck with the paths. I'm getting these errors with ~30 random middles:

• Circuit failed to be created: CHANNEL_CLOSED
• Circuit failed to be created: DESTROYED
• Proxy connection timed out: 60
• Circuit failed to be created: TIMEOUT
• No such router

When i run it with the middles from a file, i instead get this error:

• EXTENDCIRCUIT returned unexpected response code: 510