OptionsDescription should survive a property getter that throws, and skip indexers / set-only properties #590

OptionsDescription(object subject) is a diagnostic view built by calling arbitrary property getters on somebody else's configuration object. Today a single getter that throws takes the whole description with it, and anything built on top of the description goes with it.

Real-world report (JasperFx/wolverine#3740): AzureServiceBusTransport.HostName threw a NullReferenceException for credential-based connections, so a Wolverine service could never build its ServiceCapabilities snapshot, so CritterWatch never completed a handshake for that service — permanently, since the same doomed read is retried on every batch.

System.Reflection.TargetInvocationException:
   at JasperFx.Descriptors.OptionsValue.Read (OptionsValue.cs:53)
   at JasperFx.Descriptors.OptionsDescription.readProperties (OptionsDescription.cs:156)
   ...
Inner exception System.NullReferenceException:
   at Wolverine.AzureServiceBus.AzureServiceBusTransport.get_HostName

Two related holes in the same walk, both reachable from perfectly legal types:

• Indexers. Type.GetProperties() includes them, and PropertyInfo.GetValue(subject) without index arguments throws TargetParameterCountException. Wolverine.Pulsar's PulsarTransport has a this[Uri] indexer, which is enough to make it undescribable.
• Set-only properties. No getter at all; GetValue throws ArgumentException.

Proposed fix

• Skip properties where !CanRead or GetIndexParameters().Length > 0.
• Catch anything a getter throws (including from the [ChildDescription] / [DescribeAsStringArray] / [DescribeAsConfigurationState] branches) and record an OptionsValue.Unreadable(...) in its place — value "Could not be read -- <ExceptionTypeName>".
• Report the exception type only, never the message: descriptions are shipped to monitoring consoles and go to some length to keep secrets out, and exception messages habitually quote the offending configuration value (ArgumentNullException on a connection string, etc.).