R1:10cc633b88b2

Warn and continue when failing to extract pht() strings

Summary:
Ref T5267. Fixes T9643. Currently, we can not parse/extract a small fraction of strings:

- PHP files which contain obscure syntax that XHPAST can not parse.
- HEREDOCs do not have a useable `evalStatic()` behavior right now.

Emit these as warnings, but continue and generate all usable/extractable translations.

Test Plan:
```
epriestley@orbital ~/dev/phabricator $ ./bin/i18n extract . >/dev/null
Found 4,548 files...
Done.
WARNING…