fix: replace ICON_INVALID text with original error and fix readme

[sakshya73]

I was trying out this package and faced some issues so I thought to address those.

Issues: Android:

• I had an error in my AndroidManifest.xml file for the package name but in the log, I was getting "ICON_INVALID". solution - Replace "ICON_INVALID" with the original error.

ios:

• I wasn't able to change my icon and was constantly getting not able to find changeIcon because I forgot to do pod install as it wasn't mentioned in the docs. solution - added a step for pod install in the docs.

Thanks for this great package :) @skb1129 .

[sakshya73]

Could you please look into this PR? @skb1129

[skb1129]

custom error message are a way to standardise error handling, we can't rely on the system error messages for that