better error message when MAUDE_LIB not set

[mcodescu]

When the env variable MAUDE_LIB is not set, the erorr message currently displayed is

hets: fd:6: hFlush: resource vanished (Broken pipe)

[maeder]

hets -g Maude/test.maude

would simply block if MAUDE_LIB is not set (so the whole communication should be done using forkIO)

In changeset:12786 I simply check now for MAUDE_LIB

if the binary maude is not found the exitcode is 127, so maybe this case should also be checked differently.