bug: fixes bug with error out with using kompose convert with no -f (#1725)

#### What type of PR is this?

<!--
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/kind cleanup
/kind documentation
/kind feature
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/kind bug

Fixes a validation bug where if you do not provide a compose.yaml or
docker-compose.yaml it will nil point error out rather than have an
appropriate "file not found" output.

#### Which issue(s) this PR fixes:
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*Automatically closes linked issue when PR is merged.
Usage: `Fixes #<issue number>`, or `Fixes (paste link of issue)`.
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Closes https://github.com/kubernetes/kompose/issues/1719

#### Special notes for your reviewer:

Signed-off-by: Charlie Drage <charlie@charliedrage.com>

cmd/convert.go

@@ -133,7 +133,12 @@ var convertCmd = &cobra.Command{
 		}
 
 		app.ValidateFlags(args, cmd, &ConvertOpt)
-		app.ValidateComposeFile(&ConvertOpt)
+
+		// Since ValidateComposeFiles returns an error, let's validate it and output the error appropriately if the validation fails
+		err := app.ValidateComposeFile(&ConvertOpt)
+		if err != nil {
+			log.Fatalf("Error validating compose file: %v", err)
+		}
 	},
 	Run: func(cmd *cobra.Command, args []string) {