fix(frontends/lean/server): do not fail if file does not exist in 'VISIT file'

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean/server): instantiate implicit arguments with metavariables when generating FINDP and FINDG output
feat(frontends/lean/server): use '?a' instead of '?M_i' for implicit arguments when displaying FINDP and FINDG matches

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>