[cli] do not complain about "arguments must be .specs file" from non-toplevel invocations

Summary:
This error message is confusing when the user is not actually running
InferPrint, eg `infer foo`: `Load Error: file foo: arguments must be .specs
files`.

With this diff, we don't get any error for `infer foo`, which is not great
either and will need to be addressed (do we support all the python arguments in
OCaml now too and are able to turn on argument parsing errors in OCaml land?).

Reviewed By: jberdine

Differential Revision: D4397765

fbshipit-source-id: e7ca48f

infer/src/backend/InferPrint.re

@@ -9,6 +9,8 @@
  */
 open! IStd;
 
+let module CLOpt = CommandLineOption;
+
 let module Hashtbl = Caml.Hashtbl;
 
 let module L = Logging;
@@ -1190,7 +1192,8 @@ let process_summary filters formats_by_report_kind linereader stats top_proc_set
 
 let module AnalysisResults = {
   type t = list (string, Specs.summary);
-  let spec_files_from_cmdline () => {
+  let spec_files_from_cmdline () =>
+    if (Config.current_exe == CLOpt.Print) {
       /* Find spec files specified by command-line arguments.  Not run at init time since the specs
          files may be generated between init and report time. */
       IList.iter
@@ -1210,6 +1213,8 @@ let module AnalysisResults = {
       } else {
         List.rev Config.anon_args
       }
+    } else {
+      load_specfiles ()
     };
 
   /** apply [f] to [arg] with the gc compaction disabled during the execution */