Avoid blindly throwing exceptions

[johnbillion]

See #218.

bbl_get_new_term_translation_url () and Babble_Taxonomies::get_new_term_translation_url() now return a WP_Error object rather than throwing an exception (and the term links in the admin toolbar aren't built if so).

bbl_is_public_lang() and Babble_Languages:: is_public_lang() now accept a language object for the $lang_code parameter rather than throwing an exception.

Babble_Plugin:: setup() switches to a required parameter rather than throwing an exception.

get_transid() and set_transid() handle errors and cache checking more gracefully and return false (as they should) instead of throwing an exception.

[simonwheatley]

I'm still seeing two:

• [x] https://github.com/Automattic/babble/blob/b9945e0442533dfdf90e8655405d532a67bb9321/class-taxonomy.php#L1021
• [x] https://github.com/Automattic/babble/blob/b9945e0442533dfdf90e8655405d532a67bb9321/class-taxonomy.php#L371

[johnbillion]

Fixed those two remaining exceptions.

• The term re-syncing code hasn't been used since the translation workflow was introduced in #106.
• A term without a transid can now fall through so it gets one, rather than throwing an exception.