Include a list of valid enum values if an invalid enum value is detected during config loading.

[jcollie]

As discussed on Discord earlier, it would be very helpful to include a list of valid enum values in the error message if an invalid enum value is detected during config loading. For simplicity's sake it would only be added for enum values at this time but in theory this could be extended in the future to cover more types.

https://discord.com/channels/1005603569187160125/1005603569711452192/1302333119277437039

Here's a sample of what this might look like:

[mitchellh]

Accepted #2635.