Refactor the Error heriarchy

[jcouball]

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Description

This PR redesigns the custom errors defined by this gem. Here is a description of the new design from the updated README.md:

This gem raises custom errors that derive from Git::Error. These errors are
arranged in the following class heirarchy:

Error heirarchy:

Error
└── CommandLineError
    ├── FailedError
    └── SignaledError
        └── TimeoutError

Other standard errors may also be raised like ArgumentError. Each method should
document the errors it may raise.

Description of each Error class:

• Error: This catch-all error serves as the base class for other custom errors in this
  gem. Errors of this class are raised when no more approriate specific error to
  raise.
• CommandLineError: This error is raised when there's a problem executing the git
  command line. This gem will raise a more specific error depending on how the
  command line failed.
• FailedError: This error is raised when the git command line exits with a non-zero
  status code that is not expected by the git gem.
• SignaledError: This error is raised when the git command line is terminated as a
  result of receiving a signal. This could happen if the process is forcibly
  terminated or if there is a serious system error.
• TimeoutError: This is a specific type of SignaledError that is raised when the
  git command line operation times out and is killed via the SIGKILL signal. This
  happens if the operation takes longer than the timeout duration configured in
  Git.config.timeout or via the :timeout parameter given in git methods that
  support this parameter.

Git::GitExecuteError remains as an alias for Git::Error. It is considered
deprecated as of git-2.0.0.

Here is an example of catching errors when using the git gem:

begin
  timeout_duration = 0.001 # seconds
  repo = Git.clone('https://github.com/ruby-git/ruby-git', 'ruby-git-temp', timeout: timeout_duration)
rescue Git::TimeoutError => e # Catch the more specific error first!
  puts "Git clone took too long and timed out #{e}"
rescue Git::Error => e
  puts "Received the following error: #{e}"
end