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

manual-tests/static-delta-generate-crosscheck.sh

@@ -42,11 +42,11 @@ assert_streq() {
 }
 
 validate_delta_options() {
-    mkdir testrepo
-    ostree_repo_init testrepo --mode=bare-user
+    ostree --repo=testrepo init --mode=bare-user
     ostree --repo=testrepo remote add --set=gpg-verify=false local file://${repo}
     ostree --repo=${repo} static-delta generate $@ --from=${from} --to=${to}
-    ostree --repo=testrepo pull --require-static-deltas local ${branch}@${from}
+    ostree --repo=${repo} summary -u
+    ostree --repo=testrepo pull local ${branch}@${from}
     assert_streq $(ostree --repo=testrepo rev-parse ${branch}) ${from}
     ostree --repo=testrepo pull --require-static-deltas local ${branch}
     assert_streq $(ostree --repo=testrepo rev-parse ${branch}) ${to}