[lsp] Add description of how to regenerate project file to missing fidl_project.json error

Change-Id: I3b356b37d12ac08be0cf15c735b02431eeac7b3c
Reviewed-on: https://fuchsia-review.googlesource.com/c/fidl-misc/+/401473
Reviewed-by: Benjamin Prosnitz <bprosnitz@google.com>
Reviewed-by: Peter Johnston <peterjohnston@google.com>
diff --git a/fidl-lsp/main.go b/fidl-lsp/main.go
index f146c7a..a7d0b0e 100644
--- a/fidl-lsp/main.go
+++ b/fidl-lsp/main.go
@@ -49,11 +49,11 @@
 	trace := log.New(os.Stderr, "[LSP] ", log.Lshortfile)
 	logOpt := jsonrpc2.LogMessages(trace)
 
-	// fidl_project.json is generated by running
-	// `python3 scripts/gen_fidl_project.py path/to/fidl_project.json`
 	fidlProject, err := analysis.LoadFidlProject(fidlProjectPath)
 	if err != nil {
-		trace.Fatalf("Failed to parse fidl_project.json at `%s`: %s\n", fidlProjectPath, err)
+		trace.Fatalf(`Failed to parse fidl_project.json at `%s`: %s
+To regenerate, run 'fx build build/fidl:validate_fidl_project_json'.
+`, fidlProjectPath, err)
 	}
 
 	// Assume that `fidlc` and `fidl-format` are distributed in the same