[rubygems/rubygems] Make sure MissingSpecVersionError#to_s prints exception message

Gem command loading errors rely on `#to_s` on the raised exception, but
in the case of `MissingSpecVersionError` that was only the exception
name, making it printed twice and no message at all.

Before:

```
ERROR:  Loading command: install (Gem::MissingSpecVersionError)
 	Gem::MissingSpecVersionError
```

After:

```
ERROR:  Loading command: install (Gem::MissingSpecVersionError)
	Could not find 'io-wait' (>= 0.a) - did find: [io-wait-0.3.0-java]
  Checked in 'GEM_PATH=/Users/deivid/Code/rubygems/rubygems/bundler/tmp/1.1/gems/system' , execute `gem env` for more information
```

https://github.com/rubygems/rubygems/commit/d06944bb2f

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