Confirmed.
Wint64xUltimate
Version 4.0.3.3 (Build ID: 0eaa50a932c8f2199a615e1eb30f7ac74279539)

And neither is in the help of the web.

Still there in LibreOffice 4.4.x

Best regards. JBF

https://wiki.documentfoundation.org/ReleaseNotes/4.0 says: 
New spreadsheet functions IFERROR and IFNA as defined in ODF OpenFormula. tdf#56124 (Winfried Donkers, Eike Rathke)
Question is: who writes help items for new functions?

I'm using version 4.3.5.2 on Windows 7, and also could not find help on IFERROR when I searched for it.  That made me think it did not exist in LO, and I spent hours massaging a spreadsheet exported from GoogleDocs (into .ods format), replacing "=_xlfn.iferror((xxx))" entries with "if(iserror(xxx),0,xxx)" entries.

Finally, I came here to create an enhancement request and found out that IFERROR is in LO after all!

So I'm adding my vote: Whoever works on the help files, PLEASE add this documentation.

Thanks,
Tim

The help exists for English-US and for German. Therefore it is a localization problem for English UK.

Thank you for your comment, Regina.  

I had not noticed til writing this that the original issue specified "UK English help".

However, I'm in the US and to the best of my knowledge everything about my LO setup (including the help files) is "English-US".  I have now upgraded from 4.3.5.2 to 4.3.6.2, and still find nothing in the help files on IFERROR.

(In reply to Regina Henschel from comment #5)
> The help exists for English-US and for German. Therefore it is a
> localization problem for English UK.

How? For example, click on Help for IF opens https://help.libreoffice.org/scalc/SC_HID_FUNC_WENN?Language=en-US&System=WIN&Version=4.4#bm_id3152789 while click on Help for IFERROR opens empty https://help.libreoffice.org/scalc/SC_HID_FUNC_IFERROR?Language=en-US&System=WIN&Version=4.4. 
So, I don't see that help exists for English-US, neither for web nor offline help.

My fault, only ISNA and ISERROR are there, but not IFNA and IFERROR. The developer who has implemented it, should had written the help text, but everyone else can write is as well.

I know Olivier have wrote local help for a lot of functions,
https://bugs.documentfoundation.org/buglist.cgi?bug_status=UNCONFIRMED&bug_status=NEW&bug_status=ASSIGNED&bug_status=REOPENED&bug_status=RESOLVED&bug_status=VERIFIED&bug_status=CLOSED&bug_status=NEEDINFO&bug_status=PLEASETEST&email1=mariosv&email2=olivier.hallot%40documentfoundation.org&emailtype1=substring&emailtype2=substring&list_id=530469&product=LibreOffice&query_format=advanced&short_desc=localhelp&short_desc_type=allwordssubstr
but seems this was missing and I think someone more like SUMIFS(); COUNTIFS(); AVERAGEIF(); AVERAGEIFS().

Please Regina is there any way to know what functions don't have the local help?
If not, the lottery, going one by from functions wizard. I could try to find them.

By the way, through the functions wizard, there is a basic help for all functions, I think.

I was free to add Mr. Olivier.Hallot@documentfoundation.org to CC.

hook into meta-bug 80430 for missing help pages.

Added fix:

https://gerrit.libreoffice.org/gitweb?p=help.git;a=commitdiff;h=8322ff882f06c6c92e4c9d9e4bf762e06a5a754c

Please add also to 4.4.

Timur: that won’t be pushed to 4.4. It requires translation work and that branch is in string freeze.