Missing Header Raises Roo::HeaderRowNotFoundError

[reshleman]

Currently, Roo raises a generic RuntimeError with the message
"Couldn't find header row" when a matching header row is not found.

This behavior makes it difficult to rescue from the most specific error
desired, because RuntimeError is the default error class for raise.

Additionally, because Roo does not have its own library-level error
class from which all errors inherit, it is difficult to differentiate
Roo library errors from errors caused by the developer / user.

From the Ruby documentation:

It is recommended that a library should have one subclass of
StandardError or RuntimeError and have specific exception types
inherit from it. This allows the user to rescue a generic exception
type to catch all exceptions the library may raise even if future
versions of the library add new exception subclasses.

This commit adds a generic Roo::Error class which inherits from
StandardError and a specific Roo::HeaderRowNotFound class that is
raised when a matching header row could not be found.

In the future, additional subclasses of Roo::Error can be added in
order to allow users to more easily rescue the correct errors.

Note: This is not a backwards-compatible change. Users who currently
rescue RuntimeError for a missing header row will now see an error
raised.

[simonoff]

I like it!