for #1235: Use `set -euo pipefail` instead of `set -e`

As per @horazont's feedback: `-u` shouts at you if you use an undefined variable (very useful to catch typos)
and `-o pipefail` improves error handling by also failing if any of the commands in a pipe fail.

tools/validate-xep0001-conformance.sh

@@ -12,7 +12,7 @@
 #
 # requires: bash, xmllint
 
-set -e
+set -euo pipefail
 
 VALIDATION_RESULT=0