Backporting to ICU 63. This is manually cherry-picked from
commit: 693adf3987
This fixes a regression introduced by commit
b12a927c93 for issue ICU-13778.
The above commit improved the error checking in the
DateTimePatternGenerator class, adding checks for errors/failures
where there previously was none at all. This was done in order to
catch catastrophic errors like out-of-memory (OOM), and properly
report them to the caller, rather than ignoring/hiding these errors.
However, in doing so it exposed a case where the code was depending
on ignoring errors in order to fall-back to the Gregorian calendar
when the default ICU locale is set to root.
This restores the previous behavior, by allowing the error of
U_MISSING_RESOURCE_ERROR to fall-though and continue without
reporting back an error to the caller.
Note: This regression was technically introduced in ICU 63, and
also effects ICU 64 as well.
5df4d7dfd8