Fail CI if docs build throws warnings

Convert all rustdocs build warnings to errors using `-D warnings` and
exit the script with 1 to signal the error.

While we are at it use `--all-features` instead of explicit feature list
when building docs. Without this the docs build fails with:

  error: too many file operands

bitcoin/contrib/test.sh

@@ -81,7 +81,7 @@ cargo run --example taproot-psbt --features=bitcoinconsensus
 
 # Build the docs if told to (this only works with the nightly toolchain)
 if [ "$DO_DOCS" = true ]; then
-    RUSTDOCFLAGS="--cfg docsrs" cargo +nightly rustdoc --features="$FEATURES" -- -D rustdoc::broken-intra-doc-links
+    RUSTDOCFLAGS="--cfg docsrs" cargo +nightly rustdoc --all-features -- -D rustdoc::broken-intra-doc-links -D warnings || exit 1
 fi
 
 # Fuzz if told to