[covargs] Don't fail for missing modules

Some modules may be missing in the ids.txt file for legitimate reasons,
don't fail the processing, just print out a warning instead.

Change-Id: Iec0c9d6f0e5e74cb928041ba1bb2b4926b9f8e6e
diff --git a/cmd/covargs/main.go b/cmd/covargs/main.go
index 2c6b735..4f4619c 100644
--- a/cmd/covargs/main.go
+++ b/cmd/covargs/main.go
@@ -176,7 +176,8 @@
 			if ref, ok := info.ids[mod.Build]; ok {
 				moduleFiles = append(moduleFiles, ref.Filepath)
 			} else {
-				return nil, fmt.Errorf("module with build id %s not found in ids.txt file", mod.Build)
+				fmt.Fprintf(os.Stderr, "WARN: module with build id %s not found in ids.txt file", mod.Build)
+				continue
 			}
 		}