changelog: report through the log instead of printing to stdout

generate_entry's status messages become log lines, and the unmet
build-dependency diagnostics travel inside the UnmetBuildDependencies
error (its Display carries the full report) instead of being printed
to stderr by the library: the caller renders both like any other
outcome.

src/build/mod.rs

@@ -389,7 +389,9 @@ pub fn run_source_build(
         };
         let report = crate::debian::deps::check_build_depends(&ctrl, &check_opts)?;
         if !report.is_ok() {
-            eprintln!("{}", report.message());
+            // The diagnostics travel inside the error (its Display carries
+            // report.message()); main renders it and maps the type to exit
+            // status 3, like dpkg-buildpackage.
             return Err(Box::new(crate::debian::deps::UnmetBuildDependencies(
                 report,
             )));