cli: Handle invalid board paths

ci/release/changelogs/next.md

@@ -14,3 +14,4 @@
 - Fixes use of `null` in `sql_table` constraints (ty @landmaj) [#1660](https://github.com/terrastruct/d2/pull/1660)
 - Fixes elk growing shapes with width/height set [#1679](https://github.com/terrastruct/d2/pull/1679)
 - Adds a compiler error when accidentally using an arrowhead on a shape [#1686](https://github.com/terrastruct/d2/pull/1686)
+- Fixes crash when using `--watch` and navigating to an invalid board path [#1693](https://github.com/terrastruct/d2/pull/1693)

d2cli/main.go

@@ -503,7 +503,7 @@ func compile(ctx context.Context, ms *xmain.State, plugins []d2plugin.Plugin, la
 
 		board := diagram.GetBoard(boardPath)
 		if board == nil {
-			return nil, false, fmt.Errorf("Diagram with path %s not found", boardPath)
+			return nil, false, fmt.Errorf(`Diagram with path "%s" not found. Did you mean to specify a board like "layers.%s"?`, boardPath, boardPath)
 		}
 
 		boards, err := render(ctx, ms, compileDur, plugin, renderOpts, inputPath, outputPath, bundle, forceAppendix, page, ruler, board)

d2target/d2target.go

@@ -95,29 +95,24 @@ func (d *Diagram) getBoard(boardPath []string) *Diagram {
 		return d
 	}
 
+	if len(boardPath) < 2 {
+		return nil
+	}
+
 	switch head {
 	case "layers":
-		if len(boardPath) < 2 {
-			return nil
-		}
 		for _, b := range d.Layers {
 			if b.Name == boardPath[1] {
 				return b.getBoard(boardPath[2:])
 			}
 		}
 	case "scenarios":
-		if len(boardPath) < 2 {
-			return nil
-		}
 		for _, b := range d.Scenarios {
 			if b.Name == boardPath[1] {
 				return b.getBoard(boardPath[2:])
 			}
 		}
 	case "steps":
-		if len(boardPath) < 2 {
-			return nil
-		}
 		for _, b := range d.Steps {
 			if b.Name == boardPath[1] {
 				return b.getBoard(boardPath[2:])