code health: unify error format in connection_pool

After the patch all errors in the connection_pool subpackage start
with a lowercase letter [1].

1. https://github.com/golang/go/wiki/CodeReviewComments#error-strings

Part of #208

Author: oleg-jukovec

connection_pool/connection_pool.go

@@ -25,11 +25,11 @@ var (
 	ErrWrongCheckTimeout = errors.New("wrong check timeout, must be greater than 0")
 	ErrNoConnection      = errors.New("no active connections")
 	ErrTooManyArgs       = errors.New("too many arguments")
-	ErrIncorrectResponse = errors.New("Incorrect response format")
-	ErrIncorrectStatus   = errors.New("Incorrect instance status: status should be `running`")
-	ErrNoRwInstance      = errors.New("Can't find rw instance in pool")
-	ErrNoRoInstance      = errors.New("Can't find ro instance in pool")
-	ErrNoHealthyInstance = errors.New("Can't find healthy instance in pool")
+	ErrIncorrectResponse = errors.New("incorrect response format")
+	ErrIncorrectStatus   = errors.New("incorrect instance status: status should be `running`")
+	ErrNoRwInstance      = errors.New("can't find rw instance in pool")
+	ErrNoRoInstance      = errors.New("can't find ro instance in pool")
+	ErrNoHealthyInstance = errors.New("can't find healthy instance in pool")
 )
 
 /*

connection_pool/connection_pool_test.go

@@ -431,7 +431,7 @@ func TestRoundRobinStrategy_NoReplica(t *testing.T) {
 	// RO
 	_, err = connPool.Eval("return box.cfg.listen", []interface{}{}, connection_pool.RO)
 	require.NotNilf(t, err, "expected to fail after Eval, but error is nil")
-	require.Equal(t, "Can't find ro instance in pool", err.Error())
+	require.Equal(t, "can't find ro instance in pool", err.Error())
 
 	// ANY
 	args := test_helpers.ListenOnInstanceArgs{
@@ -502,7 +502,7 @@ func TestRoundRobinStrategy_NoMaster(t *testing.T) {
 	// RW
 	_, err = connPool.Eval("return box.cfg.listen", []interface{}{}, connection_pool.RW)
 	require.NotNilf(t, err, "expected to fail after Eval, but error is nil")
-	require.Equal(t, "Can't find rw instance in pool", err.Error())
+	require.Equal(t, "can't find rw instance in pool", err.Error())
 
 	// ANY
 	args := test_helpers.ListenOnInstanceArgs{

connection_pool/example_test.go

@@ -254,7 +254,7 @@ func ExampleConnectionPool_SelectAsync_err() {
 	fmt.Println("Future", 0, "Error", err)
 
 	// Output:
-	// Future 0 Error Can't find rw instance in pool
+	// Future 0 Error can't find rw instance in pool
 }
 
 func ExampleConnectionPool_Ping() {