Fix two spec warnings when using expec to raise_error without a matcher

Author: sodabrew

spec/em/em_spec.rb

@@ -63,7 +63,7 @@
             EM.stop_event_loop
           end
         end
-      }.to raise_error
+      }.to raise_error('some error')
     end
 
     context 'when an exception is raised by the client' do

spec/mysql2/statement_spec.rb

@@ -40,8 +40,7 @@
 
   it "should raise an exception without a block" do
     statement = @client.prepare 'SELECT 1'
-    statement.execute
-    expect { statement.each }.to raise_error
+    expect { statement.execute.each }.to raise_error(LocalJumpError)
   end
 
   it "should tell us the result count" do