Avoid an error when parsing files including SyntaxError

[sinsoku]

There are cases where ERB templates are saved as *.rb.
For example, devise.
refs: https://github.com/heartcombo/devise/blob/v4.9.3/lib/generators/active_record/templates/migration.rb

Running rbs prototype rb in a directory containing these files will crash due to a SyntaxError.
This commit fixes that to avoid the error and continue processing.

fixes #1414

[ksss]

LGTM.
How about to support file mode?

[sinsoku]

If a similar change is made to file mode, the output RBS will be mixed with warnings and the exit code will be 0.

$ cat a.rb
class A < <%= @superclass %>
end

$ cat b.rb
class A
end

$ git diff
diff --git lib/rbs/cli.rb lib/rbs/cli.rb
index 0a2aeaab..bb0e823f 100644
--- lib/rbs/cli.rb
+++ lib/rbs/cli.rb
@@ -996,6 +996,9 @@ EOU

         input_paths.each do |file|
           parser.parse file.read()
+        rescue SyntaxError
+          stdout.puts "⚠️  Unable to parse due to SyntaxError: `#{file}`"
+          next
         end

         writer = Writer.new(out: stdout)

$ ./exe/rbs prototype rb a.rb b.rb
⚠️  Unable to parse due to SyntaxError: `a.rb`
class A
end

I think the use case of specifying a SyntaxError file as an argument is rare, so I don't think it's necessary to support it now.
It is not too late to think about it after others raise their requests.

[soutaro]

Thanks! 👍