Improve whitespace in ExitCodeException Show instance

[9999years]

Split off of #83.

Before, ProcessConfig's Show output would include a trailing newline. This has been fixed, so that derived Show output does not include newlines in weird places.

Before:

ghci> data Foo = Foo { a :: Int, b :: ProcessConfig () () (), c :: String } deriving Show
ghci> Foo 1 (proc "echo" ["puppy"]) "doggy"
Foo {a = 1, b = Raw command: echo puppy
, c = "doggy"}

After

ghci> Foo 1 (proc "echo" ["puppy"]) "doggy"
Foo {a = 1, b = Raw command: echo puppy, c = "doggy"}

Whitespace for the ExitCodeException Show instance has also been adjusted, to place the output closer to the relevant headers.

Before:

ghci> readProcess_ $ proc "sh" ["-c", "echo this is stdout; echo this is stderr >&2; false"]
*** Exception: Received ExitFailure 1 when running
Raw command: sh -c "echo this is stdout; echo this is stderr >&2; false"
Standard output:

this is stdout
Standard error:

this is stderr

After:

*** Exception: Received ExitFailure 1 when running
Raw command: sh -c "echo this is stdout; echo this is stderr >&2; false"

Standard output:
this is stdout

Standard error:
this is stderr

Note that because trailing whitespace is not accounted for, it is still possible to get unintuitive results depending on what exactly the subprocess prints:

ghci> readProcess_ $ proc "sh" ["-c", "echo -n this is stdout; echo -n this is stderr >&2; false"]
*** Exception: Received ExitFailure 1 when running
Raw command: sh -c "echo -n this is stdout; echo -n this is stderr >&2; false"

Standard output:
this is stdout
Standard error:
this is stderr

[tomjaguarpaw]

Thanks for this.

Before tackling the issue of whitespace I would like to resolve the issue of (not) UTF-8 encoding (#86), since I expect that's causing more practical problems. I have put at an attempted resolution at #87. Could you confirm that you are content with that resolution? If so I can merge it and we can proceed to discuss whitespace.