osbuild: tweak build() to be mypy clean

This commit tweaks build() to be mypy clean without the need to
call assert. This drops the map() and instead we use the existing
dict-like access of the manifest to get the pipeline. In practise
this should not happen but lets be prepared.

Note that a small tweak for the error is needed to make it clear
what is happening.

Thanks to Simon for raising this.

osbuild/pipeline.py

@@ -518,8 +518,8 @@ class Manifest:
         """
         results = {"success": True}
 
-        for pl in map(self.get, pipelines):
-            assert pl is not None
+        for name_or_id in pipelines:
+            pl = self[name_or_id]
             res = pl.run(store, monitor, libdir, debug_break, stage_timeout)
             results[pl.id] = res
             if not res["success"]:
@@ -569,7 +569,7 @@ class Manifest:
         pl = self.get(name_or_id)
         if pl:
             return pl
-        raise KeyError(f"'{name_or_id}' not found")
+        raise KeyError(f"'{name_or_id}' not found in manifest pipelines: {list(self.pipelines.keys())}")
 
     def __iter__(self) -> Iterator[Pipeline]:
         return iter(self.pipelines.values())