Don't show missing report errors multiple times

In the diagnose report the missing download and/or install report files
could be rendered up to four times.

This was because if both files were missing, we'd still try to render
the reports individually, which would then lead to the same errors being
printed again.

.changesets/reduce-repeating-install-report-errors-in-diagnose.md

@@ -0,0 +1,5 @@
+---
+bump: "patch"
+---
+
+Render the install report errors in the diagnose CLI output fewer times. A missing download and/or install report could sometimes be displayed up to two times, in total four errors.

lib/mix/tasks/appsignal.diagnose.ex

@@ -141,13 +141,13 @@ defmodule Mix.Tasks.Appsignal.Diagnose do
     if download_parsing_error && install_parsing_error do
       do_print_parsing_error("download", report)
       do_print_parsing_error("installation", report)
-    end
-
-    if install_parsing_error do
-      do_print_download_report(report)
-      do_print_parsing_error("installation", report)
     else
-      do_print_installation_report(report)
+      if install_parsing_error do
+        do_print_download_report(report)
+        do_print_parsing_error("installation", report)
+      else
+        do_print_installation_report(report)
+      end
     end
   end