More byte-compiler warning fixes & error handling improvements

[pierre-rouleau]

The first part and some other changes of the set were already merged in bling master. The remaining include more warning fixes and error handling logic improvements:

• the fboundp calls need quoted symbols as arguments, not pound quoted.
• there might be no recent files (or the recentf.el is not loaded): account for that and report a user error.
• even if require is used to load a file, its functions and variables must be declared otherwise the byte-compiler complains when the require is inside a form.
  • For function: add a fboundp check conditional.
  • For variable: add a defvar form that does not set the initial value.

[pierre-rouleau]

@bling These are the extra byte-compiler warning fixes. The stream of commit includes some you have already merged in, sorry about that. I've cleaned up all conflicts so it should go in fine.

After this one I'll submit the bug fix for https://github.com/bling/fzf.el/issues/84 and some other small issues.