ShellPkg: Tidy for code readability

Use error handling instead of success handling.
Less indented logic is easier to read.

Cc: Zhichao Gao <zhichao.gao@intel.com>
Cc: Ray Ni <ray.ni@intel.com>

Signed-off-by: Daniel Nguyen <daniel.nguyen@arm.com>
Reviewed-by: Zhichao Gao <zhichao.gao@intel.com>
1 file changed