Fixed in the following upstream commit:
https://github.com/redhat-performance/tuned/commit/8e202b28773f8f27a12d7fb17179731b60088520

Now it should show 'script' error messages with the default verbosity level, i.e it should show the defirqaffinity error in the log like:

2018-01-19 14:39:14,570 ERROR    tuned.plugins.plugin_script: script '/usr/lib/tuned/cpu-partitioning/script.sh' error output: 'Failed to set smp_affinity for IRQ 16: [Errno 5] Input/output error'

Verified in tuned-2.10.0-2.el7.noarch using testing profile:

# cat test-profile/tuned.conf 
[script]
script=${i:PROFILE_DIR}/script.sh

# cat test-profile/script.sh 
#!/bin/sh
. /usr/lib/tuned/functions
start() {
    echo "~~ hello stderr world ~~" > /dev/stderr
    return 1
}
stop() {
    return 0
}
process $@

New and modified error messages was visible after activation of this test profile in log file:

INFO   tuned.plugins.plugin_script: calling script '/usr/lib/tuned/test-profile/script.sh' with arguments '['start']'
ERROR  tuned.plugins.plugin_script: script '/usr/lib/tuned/test-profile/script.sh' error output: '~~ hello stderr world ~~'
ERROR  tuned.plugins.plugin_script: script '/usr/lib/tuned/test-profile/script.sh' returned error code: 1
INFO   tuned.daemon.daemon: static tuning from profile 'test-profile' applied

Old output with tuned-2.9.0-1.el7.noarch

INFO   tuned.plugins.plugin_script: calling script '/usr/lib/tuned/test-profile/script.sh' with arguments '['start']'
ERROR  tuned.plugins.plugin_script: script '/usr/lib/tuned/test-profile/script.sh' error: 1, '~~ hello stderr world ~~'
INFO   tuned.daemon.daemon: static tuning from profile 'test-profile' applied

Patch was checked and it works well. Thank you.

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHBA-2018:3172