osbuild-pipeline: improve CLI interface

In the current state, osbuild-pipeline exits with random golang error,
such as goroutine failed, which is not at all helpful. This PR
introduces CLI arguments validation and helful error messages that use
the newly introduced types so that we don't waste time guessing what
was the right way to invoke this tool.

cmd/osbuild-pipeline/main.go

@@ -3,6 +3,8 @@ package main
 import (
 	"encoding/json"
 	"flag"
+	"fmt"
+	"github.com/osbuild/osbuild-composer/internal/common"
 	"io/ioutil"
 	"os"
 
@@ -12,16 +14,42 @@ import (
 )
 
 func main() {
-	var format string
+	var imageType string
 	var blueprintArg string
 	var archArg string
 	var distroArg string
-	flag.StringVar(&format, "output-format", "", "output format")
-	flag.StringVar(&blueprintArg, "blueprint", "", "blueprint to translate")
-	flag.StringVar(&archArg, "arch", "", "architecture to create image for")
-	flag.StringVar(&distroArg, "distro", "", "distribution to create")
+	flag.StringVar(&imageType, "image-type", "", "image type, e.g. qcow2 or ami")
+	flag.StringVar(&blueprintArg, "blueprint", "", "path to a JSON file containing a blueprint to translate")
+	flag.StringVar(&archArg, "arch", "", "architecture to create image for, e.g. x86_64")
+	flag.StringVar(&distroArg, "distro", "", "distribution to create, e.g. fedora-30")
 	flag.Parse()
 
+	// Print help usage if one of the required arguments wasn't provided
+	if imageType == "" || blueprintArg == "" || archArg == "" || distroArg == "" {
+		flag.Usage()
+		return
+	}
+	
+	// Validate architecture
+	if !common.ArchitectureExists(archArg) {
+		_, _ = fmt.Fprintf(os.Stderr, "The provided architecture (%s) is not supported. Use one of these:\n", archArg)
+		for _, arch := range common.ListArchitectures() {
+			_, _ = fmt.Fprintln(os.Stderr, " *", arch)
+		}
+		return
+	}
+
+	// Validate distribution
+	if !common.DistributionExists(distroArg) {
+		_, _ = fmt.Fprintf(os.Stderr, "The provided distribution (%s) is not supported. Use one of these:\n", distroArg)
+		for _, distro := range common.ListDistributions() {
+			_, _ = fmt.Fprintln(os.Stderr, " *", distro)
+		}
+		return
+	}
+	
+	// Validate image type
+
 	blueprint := &blueprint.Blueprint{}
 	if blueprintArg != "" {
 		file, err := ioutil.ReadFile(blueprintArg)
@@ -58,7 +86,7 @@ func main() {
 		panic(err.Error())
 	}
 
-	pipeline, err := d.Pipeline(blueprint, nil, checksums, archArg, format, 0)
+	pipeline, err := d.Pipeline(blueprint, nil, checksums, archArg, imageType, 0)
 	if err != nil {
 		panic(err.Error())
 	}