Odd behaviour with :silent! call maktaba#plugin#Detect()

This may not really be a Maktaba problem at all, but it's odd nonetheless.

Given the following `.vimrc`, and Vim 7.4.922:

``` viml
set nocompatible
set rtp=foo,foo.vim,bar
source maktaba/bootstrap.vim
```

(The `&rtp` directories don't need to exist.)

Run Vim with `vim -u vimrc` and then run `:silent! call maktaba#plugin#Detect()`, and the result is:

```
Error detected while processing function maktaba#plugin#Detect[2]..maktaba#plugin#GetOrInstall[18]..4_CreatePluginObject[31]..maktaba#plugin#AddonInfo:
line 8:
E484: Can't open file bar/addon-info.json

Error detected while processing function maktaba#plugin#Detect[2]..maktaba#plugin#GetOrInstall[18]..4_CreatePluginObject[31]..maktaba#plugin#AddonInfo[8]..4_EvalJSON:
line 8:
E15: Invalid expression:
E15: Invalid expression:
```

The weird part here is the first error (and in the real situation that prompted this report, that's all we see); it makes no sense to see an E484 from `maktaba#plugin#AddonInfo()`, since it surrounds everything with a `try..catch /E48[45]:/`.

Running without `:silent!` shows the real problem:

```
Error detected while processing function maktaba#plugin#Detect[2]..maktaba#plugin#GetOrInstall:
line 11:
E605: Exception not caught: ERROR(AlreadyExists): Conflict for plugin "foo": foo/ and foo.vim/

Error detected while processing function maktaba#plugin#Detect:
line 2:
E170: Missing :endfor
```

It seems wrong that calling with `:silent!` causes a different exception to be reported (or be reported at all, given that `:silent!` is supposed to suppress them). Is this just an artifact of how Vim's exception handling works, or is Maktaba doing something wrong?

(The real code I'm looking at is calling `maktaba#plugin#RegisteredPlugins()` in the context of a `:silent!` call in order to track plugin usage.)