fix resolve link for dirs with trailing /

[tejal29]

Fixes #1112

In this PR,

• wrap errors wherever possible.
• Use filepath.Clean to remove trailing '/' from path before recursively finding ancestors.
  filepath.Dir("/foo/bar/baz/") -> returns "foo/bar/baz"
• Added a unit test with the fix. Without filepath.Clean(path), the test fails

Test_resolveSymlinkAncestor$/^dir_ends_with_$/^_is_not_a_symlink$ #gosetup
=== RUN   Test_resolveSymlinkAncestor
=== RUN   Test_resolveSymlinkAncestor/dir_ends_with_/_is_not_a_symlink
--- FAIL: Test_resolveSymlinkAncestor (0.00s)
    --- FAIL: Test_resolveSymlinkAncestor/dir_ends_with_/_is_not_a_symlink (0.00s)
        resolve_test.go:255: expected result to be /tmp/830246385/var/www not /tmp/830246385/var/www/www
FAIL

See https://play.golang.org/p/TAjN36H796z

package main

import (
	"fmt"
	"path/filepath"
)

func main() {
	fmt.Println("On Unix:")

	fmt.Println(filepath.Dir("/foo/bar/baz"))
	fmt.Println(filepath.Dir("/foo/bar/baz/"))
	fmt.Println(filepath.Dir(filepath.Clean("/foo/bar/baz/")))

}

Returns:

On Unix:
/foo/bar
/foo/bar/baz
/foo/bar

[cvgw]

Add a test for "dir ends with / is a symlink" ?