validation of --base-path only directories.
FileServer:checks --base-path but only warns on problem values
FileServer --base-path validation checks are async and console warn:
FileServer>>validateBasePath clean space stat parameter;used ifNil:ifNotNil on err; reformatted warning messages.
Rewrote AmberCli.st from IDE
Merge pull request #758 from tomrake/base-path-validation

FileServer: added validation of --base-path; only existing directories should be allowed.