[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

lib/rubygems/errors.rb

@@ -30,6 +30,7 @@ module Gem
       @name        = name
       @requirement = requirement
       @extra_message = extra_message
+      super(message)
     end
 
     def message # :nodoc:
@@ -53,8 +54,8 @@ module Gem
     attr_reader :specs
 
     def initialize(name, requirement, specs)
-      super(name, requirement)
       @specs = specs
+      super(name, requirement)
     end
 
     private