fix error messages, avoid dereferencing null pointers.

fix error messages, avoid dereferencing null pointers.
- the code becomes too indented, pull it into a separate function
- add an extra hint, the current_gnode.
- specifically, variables may be expanded during
target: prereq  solving in Suff_FindDeps, this is after parsing,
not during command execution, and the only actual indication with have is
that we're resolving a prereq of.

(this ought to fix mk35, and partially solve mk34)