Don't print missing key errors when fallback is performed

[sffc]

Currently, a number of ICU4X examples print warnings such as

2022-10-24 14:21:05,150 WARN [icu_provider::error] ICU4X data error: Missing data for locale (key: segmenter/dictionary@1, request: km)

These are harmless warnings that occur as part of vertical fallback. We're going to keep getting issues about this (#2780) so we should silence these warnings if they occur during vertical fallback.

A way to fix this is for vertical fallback to set a value in the DataRequestMetadata and then to not output the missing key error if the metadata has the flag set.

[sffc]

Note that in some cases, these warnings indicate data missing from testdata. For example, if the Khmer dictionaries are missing, maybe we should go ahead and print the warning. We should also verify that Segmenter doesn't try to load dictionary data if LSTM data is present.

[sffc]

We should discuss with @makotokato and @aethanyc and then triage.

[sffc]

Maybe we want to have an annotation in the DataRequestMetadata for whether or not to print the warning. Vertical fallback can set the annotation, as well as Segmenter.