Added a few exceptions that will be thrown at the time of errors. Makes for easier debugging. Also updated README accordingly

src/Gloudemans/Shoppingcart/Exceptions/ShoppingcartInstanceException.php

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+<?php namespace Gloudemans\Shoppingcart\Exceptions;
+
+class ShoppingcartInstanceException extends \Exception {}

src/Gloudemans/Shoppingcart/Exceptions/ShoppingcartInvalidItemException.php

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+<?php namespace Gloudemans\Shoppingcart\Exceptions;
+
+class ShoppingcartInvalidItemException extends \Exception {}

src/Gloudemans/Shoppingcart/Exceptions/ShoppingcartInvalidPriceException.php

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+<?php namespace Gloudemans\Shoppingcart\Exceptions;
+
+class ShoppingcartInvalidPriceException extends \Exception {}

src/Gloudemans/Shoppingcart/Exceptions/ShoppingcartInvalidQtyException.php

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+<?php namespace Gloudemans\Shoppingcart\Exceptions;
+
+class ShoppingcartInvalidQtyException extends \Exception {}

src/Gloudemans/Shoppingcart/Exceptions/ShoppingcartInvalidRowIDException.php

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+<?php namespace Gloudemans\Shoppingcart\Exceptions;
+
+class ShoppingcartInvalidRowIDException extends \Exception {}