[issue2551078] Fix traceback caused when putting two id's into a Link html field.

New submission from John Rouillard:

In the roundup tracker, superseder is a Link type (not multilink as in
classic tracker). If you put in two link id's (1169513,2550964) you get
a traceback:

File ".../roundup/cgi/templating.py", line 886, in __getitem__
if int(self._nodeid) > 0:
ValueError: invalid literal for int() with base 10: '1169513,2550964'

This should be put inside a try/catch block to give a better message
to the user.

Maybe a message like: Property superseder is a Link and only supports 
one value."

----------
components: Web interface
messages: 6863
nosy: rouilj
priority: normal
severity: normal
status: new
title: Fix traceback caused when putting two id's into a Link html field.
type: crash

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Roundup tracker <[email protected]>
<https://issues.roundup-tracker.org/issue2551078>
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