A little quick note, when needing composite keys in your relationship in SQL XML Bulk Load the syntax is as follow:

<sql:relationship name="Detail"
                  parent="Import.Product"
                  parent-KEY="ProductID FileID"
                  child="Import.ProductDetails"
                  child-KEY="ProductID FileID"/>  

meaning they’re simply space separated in the relationship annotation. The composite key in this aspect are the columns ProductID and FileID.
As can also be seen in this annotation, I'm using a schema qualifiaction with Import, so if you have other schemas, you can simply write them in the parent/child.

For more syntax, please refer to the blog post SQL XML Bulk Load - basic XSD syntax

Seeing as I have now worked somewhat the SQL XML Bulk load, I have decided to start compiling a list of the errors I got and what I did to solve them to help myself (and others) the next time I encounter one of these errors.

The statement has been terminated.
This usually means some SQL Error has occurred within the bulk load, for example a wrong data type, truncating of data and so on.
Check the ErrorLog file for more information.

Schema: the relationship XXX referred to in the node 'YYY' is not defined.
This means – as the error says – that a relationship is missing in the schema file. Usually more often than not, this means I’ve misspelled something in my relationship definitions or where I use the relationship. Otherwise make sure your relationship is defined if it is missing.

Schema: the annotation 'field' on the empty content element 'XXX' is not supported.
The “field” annotation is set on what is a container tag. You can’t assign a field to a container tag, but it needs to be set on an element.

Schema: the parent/child table of the relationship on 'XXX' does not match.
This for me usually means that I’ve forgotten to add  the relation on the field which has a relationship, or that I’m accessing the wrong tables.

The column 'XXX' was defined in the schema, but does not exist in the database.
Well, obviously, this means that there’s a field mapping in the schema file which doesn’t match the database. Check the spelling of the field/column name in the schema file.

Invalid character value for cast specification.
This means that you’re trying to make an invalid cast, for example trying to cast a string into a number field.

Dropping a user which owns a schema will result in the error message “The database principal owns a schema in the database, and cannot be dropped”.

To solve this you can assign the schema back to its base principal with the ALTER AUTHORIZATION message, or it can be moved to another user/principal.
If for example user Test1 owns the db_owner schema and you want to drop Test1 but don’t have another user to move the schema to you can type:

ALTER AUTHORIZATION ON SCHEMA::db_owner TO db_owner  
(or for example db_datawriter if that was the case and so on)

If you wanted to move the schema to another user the syntax would be:

ALTER AUTHORIZATION ON SCHEMA::<theSchema> TO userName