Taxonomy field 'column' should be 'tid' instead of 'value'

Thanks for tackling this, c960657.
Forum heavily uses taxonomy, and should be updated accordingly
forum_node_prepare(), forum_node_validate(), forum_node_presave()...
Beware that we don't want to change stuff like 'title' => $object->title[FIELD_LANGUAGE_NONE][0]['value'],, of course.

It's very possible that some taxonomy or forum tests need to be updated to reflect the change.

About taxonomy_field_widget_error(): Mmh, actually that function should simply be

function taxonomy_field_widget_error($element, $error) {
  form_error($element, $error['message']);
}

because taxonomy.module implements only one widget itself - so that other code branch is never executed.

Good call on the variable renames in a few foreach() loops.

About the taxo test: I think the pattern used elsewhere to test whether an exception happens or not is:

try {
  bla();
  $this->pass(t('bla() did not raise an exception.'));
}
catch (Exeption $e) {
  $this->fail(t('bla() did not raise an exception.'));
}
<code>
or 
<code>
try {
  bla();
  $this->fail(t('bla() raised an exception.'));
}
catch (Exeption $e) {
  $this->pass(t('bla() raised an exception.'));
}

+ the scope of $e is limited to the catch (Exeption $e) { ... } block, so no need to reset $e between two try / catch sequences.

Other than that, looks ready to me.

Cool ! RTBC.

The forum.module hunks were left out of the commit, forum tests fail on HEAD.

Thx Dries. Resetting title and priority for posterity.