api invalid address checks

[raedah]

trafficstars

https://explorer.dcrdata.org/api/address/sdfdsfdsfdsdjflkjsjkfjlkdsjflksdjflkjdsf/count/5/raw

too many address

https://explorer.dcrdata.org/api/address/sdfdsfdsfdsdjflkjsjkfjlkdsjflkjdsf/count/5/raw

invalid address

Address validity checks only seem to take account of the string length. Passing an address that is invalid but is a long string will result in an incorrect error msg, 'too many address'.

[ukane-philemon]

https://explorer.dcrdata.org/api/address/sdfdsfdsfdsdjflkjsjkfjlkdsjflksdjflkjdsf/count/5/raw

too many address

This now returns Unprocessable entity. This new response closes this issue?