ShellPkg: Properly set lasterror environment variable

ShellPkg: Properly set lasterror environment variable
- Set lasterror value based on platform width
- Use “lasterror” rather than “Lasterror” for variable name to match Shell Specification
- Set lasterror value when a binary or script file is not found

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Chris Phillips <chrisp@hp.com>
Reviewed-by: Jaben Carsey <Jaben.Carsey@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@14816 6f19259b-4bc3-4df7-8a09-765794883524