Add type encoding checks for NSArray elements before encoding.

[copybara-service[bot]]

Add type encoding checks for NSArray elements before encoding.

With strict mode available in EarlGrey, there will be more users always sending NSArray by-value. One caveats from that is, NSArray doesn't always conform to NSCoding. The failure will happen if any element of the array does not conform to NSCoding. Right now, the encoding error doesn't have useful information, but just saying " does not conform to encodeWithCoder:".

This change polishes the error message in this case, when failure happens, users will know:

• failure happens during the eDO remote invocation.
• which array causes the issue.
• which object causes the issue.