[lldb][Target][NFC] Update error messages according to LLVM-coding style (#181795)
The [LLVM coding style
guide](https://llvm.org/docs/CodingStandards.html#error-and-warning-messages)
says:
> start the first sentence with a lowercase letter, and finish the last
sentence
> without a period
This patch updates the erorr messages added in
https://github.com/llvm/llvm-project/pull/179208 accordingly.
(addresses
https://github.com/llvm/llvm-project/pull/179208#issuecomment-3912273053)