In the wake of r7415, let's have some better error reporting.

In the wake of r7415, let's have some better error reporting.
Instead of passing -1 to its gotdata and sentdata callbacks on
error, winhandl.c will now pass the negation of the Windows error
number; and the Plink front end will now format that into an error
message and pass it on to the user.

[originally from svn r7416]
[r7415 == 702a92ceb848394f722292cd4a54c414d4e0be67]