Show tier in buildbot failure message

[hugovk]

Fixes #582.

I don't have a good way of testing this, I did so by putting this at the end of builders.py and ran make check:

print(get_builder_tier("AMD64 Fedora Stable LTO + PGO"))
print(get_builder_tier("AMD64 Fedora Stable Clang Installed 3.x"))
print(get_builder_tier("aarch64 Fedora Stable Clang Installed 3.x"))
print(get_builder_tier("aarch64 Fedora Stable Clang Installed 3.11"))
print(get_builder_tier("aarch64 Fedora Stable Clang Installed 3.9"))
print(get_builder_tier("aarch64 RHEL8 LTO 3.13"))
print(get_builder_tier("AMD64 RHEL8 FIPS Only Blake2 Builtin Hash 3.x"))
print(get_builder_tier("PPC64LE Fedora Rawhide Clang Installed 3.x"))

And got:

tier-1
tier-2
tier-2
tier-2
tier-2
tier-2
no tier
tier-3

[picnixz]

Maybe we should have a data structure holding those? like STABLE_TIER_LIST = {TIER_1: STABLE_BUILDERS_TIER_1, ... and UNSTABLE_TIER_LIST = {...}?