Merge #16224: gui: Bilingual GUI error messages

18bd83b1fee2eb47ed4ad05c91f2d6cc311fc9ad util: Cleanup translation.h (Hennadii Stepanov)
e95e658b8ec6e02229691a1941d688e96d4df6af doc: Do not translate technical or extremely rare errors (Hennadii Stepanov)
7e923d47ba9891856b86bc9f718cf2f1f773bdf6 Make InitError bilingual (Hennadii Stepanov)
917ca93553917251e0fd59717a347c63cdfd8a14 Make ThreadSafe{MessageBox|Question} bilingual (Hennadii Stepanov)
23b9fa2e5ec0425980301d2eebad81e660a5ea39 gui: Add detailed text to BitcoinGUI::message (Hennadii Stepanov)

Pull request description:

  This is an alternative to #15340 (it works with the `Chain` interface; see: https://github.com/bitcoin/bitcoin/pull/15340#issuecomment-502674004).
  Refs:
  - #16218 (partial fix)
  - https://github.com/bitcoin/bitcoin/pull/15894#issuecomment-487947077

  This PR:
  - makes GUI error messages bilingual: user's native language + untranslated (i.e. English)
  - insures that only untranslated messages are written to the debug log file and to `stderr` (that is not the case on master).

  If a translated string is unavailable only an English string appears to a user.

  Here are some **examples** (updated):

  ![Screenshot from 2020-04-24 17-08-37](https://user-images.githubusercontent.com/32963518/80222043-e2458780-864e-11ea-83fc-197b7121dba5.png)

  ![Screenshot from 2020-04-24 17-12-17](https://user-images.githubusercontent.com/32963518/80222051-e5407800-864e-11ea-92f7-dfef1144becd.png)

  * `qt5ct: using qt5ct plugin` message is my local environment specific; please ignore it.

  ---

  Note for reviewers: `InitWarning()` is out of this PR scope.

ACKs for top commit:
  Sjors:
    re-tACK 18bd83b1fee2eb47ed4ad05c91f2d6cc311fc9ad
  MarcoFalke:
    ACK 18bd83b1fee2eb47ed4ad05c91f2d6cc311fc9ad 🐦

Tree-SHA512: 3cc8ec44f84403e54b57d11714c86b0855ed90eb794b5472e432005073354b9e3f7b4e8e7bf347a4c21be47299dbc7170f2d0c4b80e308205ff09596e55a4f96

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