Update error message to mention why adding an asset fails

[srirambv]

Currently when you try to add a custom asset that is already added/listed as visible asset it throws an error that says Failed to add custom token, please try again but doesn't tell why its failed. We should add details so its clear as to why it can't be added.

cc: @jamesmudgett @nuo-xu @SergeyZhukovsky @Douglashdaniel

[HEagle18]

@Douglashdaniel would you be able to take this ?

I just redid the flow.

• Added OKB as a custom token
• it was successful
• tried to readd it by entering contract address
• it just skips to next main screen
• if you return, you get 'failed to add custom token'

Could we add 'token already added to portfolio'

[srirambv]

Android specific issue #28670 is logged, removing OS/Android label

[srirambv]

Verification passed on

Brave	1.49.108 Chromium: 110.0.5481.104 (Official Build) beta (64-bit)
Revision	46de4a7f41979e829b430bc1ee30ef483aa227ac-refs/branch-heads/5481_77@{#19}
OS	Linux

• Verified steps from brave/brave-core#16761
• Verified trying to add an existing token shows the error message and disabled Add button

21296.mp4

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Verification passed on

Brave	1.49.108 Chromium: 110.0.5481.104 (Official Build) beta (64-bit)
Revision	46de4a7f41979e829b430bc1ee30ef483aa227ac-refs/branch-heads/5481_77@{#19}
OS	Windows 11 Version 22H2 (Build 22621.755)

• Verified steps from brave/brave-core#16761
• Verified trying to add an existing token shows the error message and disabled Add button

21296.mp4

---

Verification passed on

Brave	1.49.108 Chromium: 110.0.5481.104 (Official Build) beta (arm64)
Revision	46de4a7f41979e829b430bc1ee30ef483aa227ac-refs/branch-heads/5481_77@{#19}
OS	macOS Version 13.0 (Build 22A380)

• Verified steps from brave/brave-core#16761
• Verified trying to add an existing token shows the error message and disabled Add button

21296.mov