OPC 10000-4 v1.05.07 §5.12.2: one per result, in the same order, when the server returned any; empty
otherwise (the server had none, or returnDiagnostics asked for no operation-level field).
The strings each DiagnosticInfo points at in the ResponseHeader stringTable (OPC 10000-4 §7.12), one per entry
of diagnosticInfos; null where there is none. symbolicId and namespaceUri need the operation-level
SymbolicId bit (0x20) in returnDiagnostics, localizedText and locale the LocalizedText bit (0x40).
The results of a Call, with the DiagnosticInfo the server returned for each result's statusCode.