Put the rest of the unsolicited messages behind projectile-verbose

[bbatsov]

An audit of all 60 message call sites in projectile.el.

The good news is that the line projectile-verbose was drawing is the right
one - Projectile stays quiet about what it does off its own bat, and still
answers for what you asked it to do. It just wasn't drawn consistently: it
gated nine sites and missed five that belong on the same side of it.

• Caching a file the find-file hook picked up. Fires for every newly
  cached file you open, which is the most frequent of the lot.
• A search path entry that no longer exists. The automatic scan runs once a
  session, so a stale entry announces itself on every startup.
• An indexing command that exited non-zero but produced usable output, in
  both the sync and async runners. The code's own comment already said to
  "mention it quietly"; the detail is in *projectile-files-errors* either way.
• A session file from an older format version. projectile-session-restore-all
  can walk a directory full of these at startup.

Two of those are also commands in their own right, so they use the pattern
already established in projectile-discard-command-cache: quiet from a hook,
still speaking when you invoke them. There are specs for both sides of that.

Deliberately left alone:

• Projectile is initializing cache for ... - it looks like noise in a test log,
  but it is the only warning that synchronous native indexing is about to take
  a while. The async path has a progress reporter; native has this.
• Every message that is a command answering for itself. projectile-invalidate-cache
  saying nothing would leave you wondering whether it ran, and there is a spec
  pinning that it still speaks.

The option's docstring and the manual now describe what it actually covers,
rather than "echo messages that are not errors".