Add check for valid keys in `importprivkey` #3593

The base58 armoring was checked, but not the resulting private key, which could be out of range. Fix this by adding a check.

check errors due to wrong encoding

importprivkey foo
error: {"code":-5,"message":"Invalid private key encoding"}

=0x00....

importprivkey cMahea7zqjxrtgAbB7LSGbcQUr1uX1ojuat9jZodMN87J7g8rY9t
error: {"code":-5,"message":"Private key outside allowed range"}

=0xFF....

importprivkey cWALDjUu1tszsCBMjBjL4mhtjHDZMS5U52WHJ29iqqspZx2Er6J2
error: {"code":-5,"message":"Private key outside allowed range"}

=n-1

importprivkey cWALDjUu1tszsCBMjBjL4mhYj2wHUWYDR8Q8aSjLKzjkW5eBtpzu
(ok)

=n

importprivkey cWALDjUu1tszsCBMjBjL4mhYj2wHUWYDR8Q8aSjLKzjkWaXMLRaY
error: {"code":-5,"message":"Private key outside allowed range"}

Automatic sanity-testing: PASSED, see http://jenkins.bluematt.me/pull-tester/c7f933259623c87a9a3d116cc9bcfd75deb497da for binaries and test log. This test script verifies pulls every time they are updated. It, however, dies sometimes and fails to test properly. If you are waiting on a test, please check timestamps to verify that the test.log is moving at http://jenkins.bluematt.me/pull-tester/current/ Contact BlueMatt on freenode if something looks broken.

ACK, thanks.

ACK