Add newline character after error message

[JOE1994]

Currently, error messages emitted to stderr don't end with a newline character.

• example

  lonelyjoe@lonelyjoe-desktop:~/workspace/Rust_abstractions/basicmath_large$ cargo asm
  [ERROR]: cargo asm could not find any output files!lonelyjoe@lonelyjoe-desktop:~/workspace/Rust_abstractions/basicmath_large$ 
  lonelyjoe@lonelyjoe-desktop:~/workspace/Rust_abstractions/basicmath_large$ 

This commit adds a newline character at the end of the printed error message, to slightly improve user experience.

• example after this commit

  lonelyjoe@lonelyjoe-desktop:~/workspace/Rust_abstractions/basicmath_large$ cargo asm
  [ERROR]: cargo asm could not find any output files!
  lonelyjoe@lonelyjoe-desktop:~/workspace/Rust_abstractions/basicmath_large$ 
  lonelyjoe@lonelyjoe-desktop:~/workspace/Rust_abstractions/basicmath_large$ 

Thank you for reviewing this PR :+1:

p.s. I also made one minor change that moves branch-invariant parts out of the if-else block.