osbuild: also print what export is availalble when one is not found

The current error message when an export is not found could be
improved by printing what exports are actually availalble to make
it easier for the user to e.g. spot typos.

osbuild/main_cli.py

@@ -138,8 +138,9 @@ def osbuild_cli() -> int:
     exports = set(args.export)
     unresolved = [e for e in exports if e not in manifest]
     if unresolved:
+        available = list(manifest.pipelines.keys())
         for name in unresolved:
-            print(f"Export {vt.bold}{name}{vt.reset} not found!")
+            print(f"Export {vt.bold}{name}{vt.reset} not found in {available}")
         print(f"{vt.reset}{vt.bold}{vt.red}Failed{vt.reset}")
         return 1