fix: log err when loading mibs

[reimda]

Log the error when loading mibs. This will give us more information in logs and hopefully clear up why users are seeing the "Couldn't stat target" log message.

[MyaLongmire]

This will be super useful!

[telegraf-tiger]

Download PR build artifacts for linux_amd64.tar.gz, darwin_amd64.tar.gz, and windows_amd64.zip.
Downloads for additional architectures and packages are available below.

☺️ This pull request doesn't significantly change the Telegraf binary size (less than 1%)

📦 Click here to get additional PR build artifacts

Artifact URLs

DEB	RPM	TAR GZ	ZIP
amd64.deb	aarch64.rpm	darwin_amd64.tar.gz	windows_amd64.zip
arm64.deb	armel.rpm	darwin_arm64.tar.gz	windows_i386.zip
armel.deb	armv6hl.rpm	freebsd_amd64.tar.gz
armhf.deb	i386.rpm	freebsd_armv7.tar.gz
i386.deb	ppc64le.rpm	freebsd_i386.tar.gz
mips.deb	riscv64.rpm	linux_amd64.tar.gz
mipsel.deb	s390x.rpm	linux_arm64.tar.gz
ppc64el.deb	x86_64.rpm	linux_armel.tar.gz
riscv64.deb		linux_armhf.tar.gz
s390x.deb		linux_i386.tar.gz
		linux_mips.tar.gz
		linux_mipsel.tar.gz
		linux_ppc64le.tar.gz
		linux_riscv64.tar.gz
		linux_s390x.tar.gz
		static_linux_amd64.tar.gz

[Hipska]

Example output from instances where they have mib loading problems:

Couldn't stat target /usr/share/snmp/mibs: lstat /usr/share/snmp/mibs/usr/share/snmp/mibs>

Not from me, but as reported in #10674 (comment) (it seems like the message is truncated)

[ericjohncarlson]

I also ran this binary (Telegraf 1.22.0-414f7780 (git: snmp-translate-log-err 414f7780)) and received the same output as Hipska:

2022-02-28T13:52:04Z W! [inputs.snmp] Couldn't stat target /usr/share/snmp/mibs: lstat /usr/share/snmp/mibs/usr/share/snmp/mibs: no such file or directory
2022-02-28T13:52:04Z W! [inputs.snmp] Couldn't stat target /usr/share/snmp/mibs: lstat /usr/share/snmp/mibs/usr/share/snmp/mibs: no such file or directory