Improve some `brew install` messaging.

Improve the messaging around `brew install` when there's a possible user
action such as an `upgrade` or `link` and don't tell people to
`install --force` when it's unnecessary.

While I did this, tweak the output and function usage in a couple of
related places.

Some example output before this change:
```
Warning: openssl is a keg-only and another version is linked to opt.
Use `brew install --force` if you want to install this version
Warning: mysql@5.6 is a keg-only and another version is linked to opt.
Use `brew install --force` if you want to install this version
Warning: analog-6.0_1 already installed
Warning: bash-completion@2-2.5 already installed, it's just not linked.
```

Some example output after this change:
```
Error: openssl 1.0.2k is already installed
To upgrade to 1.0.2l, run `brew upgrade openssl`
Warning: mysql@5.6 5.6.36_1 is already installed
Warning: analog 6.0_1 is already installed
Warning: bash-completion@2 2.5 is already installed, it's just not linked.
You can use `brew link bash-completion@2` to link this version.
```

1 files changed, 1 insertions, 1 deletions

diff --git a/Library/Homebrew/exceptions.rb b/Library/Homebrew/exceptions.rb
index 6751b2224..5001c550d 100644
--- a/Library/Homebrew/exceptions.rb
+++ b/Library/Homebrew/exceptions.rb
@@ -325,7 +325,7 @@ class FormulaConflictError < RuntimeError
 
   def message
     message = []
-    message << "Cannot install #{formula.full_name} because conflicting formulae are installed.\n"
+    message << "Cannot install #{formula.full_name} because conflicting formulae are installed."
     message.concat conflicts.map { |c| conflict_message(c) } << ""
     message << <<-EOS.undent
       Please `brew unlink #{conflicts.map(&:name)*" "}` before continuing.