Fix error reporting when parsing genrule cmd

Report errors inside the android.Expand lambda and don't return an
error so that processing can continue and multiple errors can be
reported.  Check for errors (for example incorrect syntax of variables)
immediately after Expand returns.  Avoids a misleading error message:
specified depfile=true but did not include a reference to '${depfile}' in cmd

Bug: 111219250
Test: m checkbuild
Change-Id: Id9a16c1609f5fd9345bfa1a2191261cff72fd382