Sync warn modal needs refactoring

Description

Sync warn modal needs refactoring

Steps to Reproduce

1. Enable sync on beta with --enable-brave-sync
2. Click on I have an existing Sync code
3. Click Setup Sync without entering any code or device name, shows the warn message which is just plain text (see screenshot)

Actual result:

Expected result:

Should show proper error message which is refactored

Reproduces how often:

Easy

Brave version (brave://version info)

Brave	0.57.6 Chromium: 71.0.3578.31 (Official Build) beta (64-bit)
Revision	c88fdf2a4ce19a713615ca4fbde7a0d0b5fe2363-refs/branch-heads/3578@{#427}
OS	All

Reproducible on current release:

• Does it reproduce on brave-browser dev/beta builds? Yes on beta build

Website problems only:

• Does the issue resolve itself when disabling Brave Shields?
• Is the issue reproducible on the latest version of Chrome?

Additional Information

#2126 also needs a warning similar to this if a device name is mandatory @rebron @bbondy @kjozwiak added to 1.x backlog as this is just a refactor which isn’t priority but if needed please add it to 0.57.x milestone.

[cezaraugusto]

ya correct

[srirambv]

Verification passed on

Brave	0.58.12 Chromium: 71.0.3578.80 (Official Build) (64-bit)
Revision	2ac50e7249fbd55e6f517a28131605c9fb9fe897-refs/branch-heads/3578@{#860}
OS	Linux

Verification passed on

Brave	0.59.8 Chromium: 71.0.3578.98 (Official Build) beta (64-bit)
Revision	15234034d19b85dcd9a03b164ae89d04145d8368-refs/branch-heads/3578@{#897}
OS	Windows

Verified passed with

Brave	0.58.12 Chromium: 71.0.3578.80 (Official Build) (64-bit)
Revision	2ac50e7249fbd55e6f517a28131605c9fb9fe897-refs/branch-heads/3578@{#860}
OS	Mac OS X

• Verified STR and test plan in https://github.com/brave/brave-core/pull/1019. Follow up issues #2575 and #2577 logged.