If you inherit useradd and add a package to USERADD_PACKAGES but don't set USERADD_PARAM or GROUPADD_PARAM, you get this:

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ERROR: Unable to parse /home/paul/poky/meta-oe/meta-oe/recipes-support/mysql/mysql5_5.1.40.bb                                                                                 | ETA:  --:--:--
Traceback (most recent call last):
  File "/home/paul/poky/poky/bitbake/lib/bb/parse/ast.py", line 335, in finalize(fn='/home/paul/poky/meta-oe/meta-oe/recipes-support/mysql/mysql5_5.1.40.bb', d=<bb.data_smart.DataSmart object at 0x152f910>, variant=None):
             code.append("%s(d)" % funcname)
    >    bb.utils.better_exec("\n".join(code), {"d": d})
         bb.data.update_data(d)
  File "/home/paul/poky/poky/bitbake/lib/bb/utils.py", line 343, in better_exec(code=<code object <module> at 0xfa0ad50, file "<code>", line 1>, context={'d': <bb.data_smart.DataSmart object at 0x152f910>}, text='__anon_635__home_paul_poky_poky_meta_classes_base_bbclass(d)\n__anon_227__home_paul_poky_poky_meta_classes_package_bbclass(d)\n__anon_1134__home_paul_poky_poky_meta_classes_package_rpm_bbclass(d)\n__anon_917__home_paul_poky_poky_meta_classes_insane_bbclass(d)\n__anon_20__home_paul_poky_poky_meta_classes_debian_bbclass(d)\n__anon_27__home_paul_poky_poky_meta_classes_devshell_bbclass(d)\n__anon_76__home_paul_poky_poky_meta_classes_sstate_bbclass(d)\n__anon_124__home_paul_poky_poky_meta_classes_siteinfo_bbclass(d)\n__anon_53__home_paul_poky_poky_meta_classes_update_rc_d_bbclass(d)\n__anon_168__home_paul_poky_poky_meta_classes_useradd_bbclass(d)', realfile='<code>'):
         try:
    >        exec(code, _context, context)
         except Exception as e:
  File "<code>", line 10, in <module>
  File "__anon_168__home_paul_poky_poky_meta_classes_useradd_bbclass", line 2, in __anon_168__home_paul_poky_poky_meta_classes_useradd_bbclass(d=<bb.data_smart.DataSmart object at 0x152f910>)
  File "useradd.bbclass", line 9, in update_useradd_after_parse(d=<bb.data_smart.DataSmart object at 0x152f910>)
FuncFailed: Function failed: /home/paul/poky/meta-oe/meta-oe/recipes-support/mysql/mysql5_5.1.40.bb inherits useradd but doesn't set USERADD_PARAM or GROUPADD_PARAM for package mysql5-server
--------- snip ----------

We only need to see the error here and not the traceback which is rather difficult to read. It could be that raising bb.build.FuncFailed is the wrong thing to be doing here.