Cleanup: Removed <threadpool> completly.

This adds a error level log line to the block in case unknown nodes are used
as we do for other blocks already.

Closes: #2085
1 job for master in 13 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
failed #540
docker alpine
build

00:00:13

 
Name Stage Failure
failed
build Build There is an unknown failure, please try again
  openssl-dev-1.0.2p-r0:
conflicts:
libressl-dev-2.7.4-r0[pc:libcrypto=1.0.2p]
libressl-dev-2.7.4-r0[pc:libssl=1.0.2p]
libressl-dev-2.7.4-r0[pc:openssl=1.0.2p]
satisfies: world[openssl-dev]
libssh2-dev-1.8.0-r3[pc:libcrypto]
libssh2-dev-1.8.0-r3[pc:libssl]
ERROR: Job failed: exit code 4