Before, an export format with an invalid enum value (for example in the
Encoding id) would crash Ardour with:
unhandled exception (type std::exception) in signal handler:
what: unknown enumerator FOO in PBD::EnumWriter
That kind of error can happen if a new type is introduced and users
switch back to versions without it.
Instead, catch such exceptions while loading a format, log an error, and
skip the format - similar to how other format loading errors are
handled.
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