removed exception catching in recursive_subspecs

see [this
discussion](https://github.com/CocoaPods/CocoaPods/commit/6bedfe0baac10e4e6ff30968b18acd111c143e1e#commitcomment-1212777)
for details

lib/cocoapods/specification.rb

@@ -131,16 +131,12 @@ module Pod
 
     def recursive_subspecs
       unless @recursive_subspecs
-        begin
-          mapper = lambda do |spec|
+        mapper = lambda do |spec|
             spec.subspecs.map do |subspec|
               [subspec, *mapper.call(subspec)]
             end.flatten
           end
           @recursive_subspecs = mapper.call self
-        rescue StandardError
-          @recursive_subspecs = []
-        end
       end
       @recursive_subspecs
     end