Don't swallow builder error messages in BuildTable

[glittershark]

If builder->Add sets _status to a failing Status, previously BuildTable would just continue looping - and the previous invocation would see seen_error() set to true and return an opaque "Writer has previous error" status, without any information on what that previous error was. This commit changes the iterator loop in BuildTable to fail-fast, so that the status returned always has the first error message.

[glittershark]

looking at the test failure, it kinda seems to me like the test might have been wrong all along? But I'm not familiar enough with the test to say that confidently - would appreciate someone with more context to take a look at that.