bundle2: move handler validation out of processpart
As part of refactoring bundle part processing let's move handler validation to
its own function.
Differential Revision: https://phab.mercurial-scm.org/D707
# See http://EditorConfig.org for the specification
root = true
[*.py]
indent_size = 4
indent_style = space
trim_trailing_whitespace = true
[*.{c,h}]
indent_size = 8
indent_style = tab
trim_trailing_whitespace = true