[NFC] Refactor and improve comments in CommandObjectTarget

Made small improvements while debugging through
CommandObjectTarget::AddModuleSymbols.

1.  Refactored error case for an early out, reducing the indentation of
the rest of this long function.
2.  Clarified some comments by correcting spelling and punctuation.
3.  Reduced duplicate code at the end of the function.

Tested with `ninja check-lldb`

Differential Review: https://reviews.llvm.org/D70458