??! is a trigraph that translates to
|. So it says:
!ErrorHasOccured() || HandleError();
which, due to short circuiting, is equivalent to:
if (ErrorHasOccured()) HandleError();
Guru of the Week (deals with C++ but relevant here), where I picked this up.
Possible origin of trigraphs or as @DwB points out in the comments it's more likely due to EBCDIC being difficult (again). This discussion on the IBM developerworks board seems to support that theory.
From ISO/IEC 9899:1999 §126.96.36.199, footnote 12 (h/t @Random832):
The trigraph sequences enable the input of characters that are not defined in the Invariant Code Set as described in ISO/IEC 646, which is a subset of the seven-bit US ASCII code set.