lib: Minor static delta fixes

lib: Minor static delta fixes

First, the manual crosscheck script bitrotted; it got caught up
in the "use libtest repo creation wrapper" bit, and also it
seems like at some point `pull --require-static-deltas` changed
meaning when dealing with `file:///` repos.  I have more work to
unwind that.

Next, I'm seeing a delta failure which looks like a static delta
miscompilation with rollsums; change the compiler to print out
the source object too, which helped me debug this.

And finally in the processing code, fix incorrect error prefixing, which was
misleading.

Closes: #1311
Approved by: ashcrow