Fix Ethereum DApp error response formatting

[StephenHeaps]

Description:

WebKit prefixes Error: when responding to script messages with an error. Similar to Solana, we need to strip this prefix so the json formatting is preserved.

ref: brave/brave-talk#882

Steps to Reproduce

1. Visit Brave Talk with a Premium account
2. Connect your wallet to Brave Talk
3. Tap Host a Web3 Call
4. Tap cancel on the signature request
5. Observe poorly formatted error message as the DApp could not parse the response.

Alternative steps without Brave Talk:

1. Visit https://metamask.github.io/test-dapp/
2. Connect your wallet to the test DApp
3. Tap Sign under Personal Sign.
4. Tap cancel on the signature request
5. Observe ERROR: ("CODE": 4001, 'MESSAGE": "THE USER REJECTED THE REQUEST.).
   • Expected: ERROR: THE USER REJECTED THE REQUEST

Actual result:

Simulator.Screen.Recording.-.iPhone.14.Pro.-.2023-05-04.at.14.07.45.mp4

Expected result:

Screenshot from Android when correct error response is given for cancel/reject:

Reproduces how often: [Easily reproduced, Intermittent Issue]

Brave Version:

• Can you reproduce this issue with the most recent build from TestFlight?
• Can you reproduce this issue with the previous version of the current build from TestFlight?
• Can you reproduce this issue with the current build from AppStore?

Device details:

Website problems only:

• did you check with Brave Shields down?
• did you check in Safari/Firefox (WkWebView-based browsers)?

Additional Information

[srirambv]

Verification passed on the following devices running 1.51 (2.5.5.18)

• Verified steps from #7393
• Verified cancel sign message shows proper response message instead of json response

iPhone 7+ (iOS 15.7.3)	iPhone 13 (iOS 16.4.1)	iPad Pro (16.4 Beta 4)
7392-iPhone.7+.MP4	7392-iPhone.13.MP4	7392-iPad.Pro.MP4