NullRef in IExternalStringResources.TryGetErrorResource

#2514 removed a lock (to avoid a horrible performance problem from contention) - but this means now we're reading a (non-concurrent) dictionary outside of the lock. This is manifesting as race condition / non-deterministic null-ref exceptions here:

https://github.com/microsoft/Power-Fx/blob/bcc750b19172ce712d75e50019f50c64d0c5d784/src/libraries/Microsoft.PowerFx.Core/Localization/StringResources.cs#L234

TryGet succeeds and hands back a null localizedErrorResources - likely due to a race (accessing a dictionary from multiple threads)

Naive fix is to protect this with a reader-writer lock.

  Message: 
System.NullReferenceException : Object reference not set to an instance of an object.

  Stack Trace: 
Dictionary`2.FindEntry(TKey key)
Dictionary`2.TryGetValue(TKey key, TValue& value)
IExternalStringResources.TryGetErrorResource(ErrorResourceKey resourceKey, ErrorResource& resourceValue, String locale)
ErrorUtils.GetLocalizedErrorContent(ErrorResourceKey errKey, CultureInfo locale, ErrorResource& errorResource)
BaseError.ctor(IDocumentError innerError, Exception internalException, DocumentErrorKind kind, DocumentErrorSeverity severity, ErrorResourceKey errKey, Span textSpan, IEnumerable`1 sinkTypeErrors, Object[] args)
BaseError.ctor(IDocumentError innerError, Exception internalException, DocumentErrorKind kind, DocumentErrorSeverity severity, ErrorResourceKey errKey, Object[] args)
TexlError.ctor(TexlNode node, DocumentErrorSeverity severity, ErrorResourceKey errKey, Object[] args)

Start in src/libraries/Microsoft.PowerFx.Core/Localization/StringResources.cs at line 234 and review the lock removal referenced in #2514. Investigate concurrent access through IExternalStringResources.TryGetErrorResource and verify that concurrent calls no longer produce a null localizedErrorResources or NullReferenceException.