Only delete cached gem when it's corrupted

Rescuing all errors here might end up hiding other errors if the
deletion of the cached gem itself raises an error for some reason. Let's
be more conservative.

bundler/lib/bundler/source/rubygems.rb

@@ -162,7 +162,7 @@ def install(spec, opts = {})
           begin
             s = Bundler.rubygems.spec_from_gem(path, Bundler.settings["trust-policy"])
             spec.__swap__(s)
-          rescue StandardError
+          rescue Gem::Package::FormatError
             Bundler.rm_rf(path)
             raise
           end