This is a try-catch wrapper. It returns a list with
four elements:
$value- The value of the evaluated expression, or NULL if an error was thrown and execution did not complete
$messages- A list of
message objects, each being a message thrown during the execution, in order
$warnings- A list of
condition objects, each being a message thrown during the execution, in order
$error- NULL if there was no error, otherwise the
error object, which can then be passed to getTraceback
to retrieve the error
The elements of the $messages, $warnings and $error are all actually two-element lists, the first being
the condition object itself (named $message, $warning or $error) and the second begin the call stack
as returned by sys.calls() and named $calls.
The class of this object is set as "AnalysisPageValueWithConditions"