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Error XD0030 covers the general condition of step failure. "It is a dynamic error (err:XD0030) if a step is unable or incapable of performing its function. This is a general error code for “step failed” (e.g., if the input isn't of the expected type or if attempting to process the input causes the implementation to abort). Users and implementors who create extension steps are encouraged to use this code for general failures." Does this mean that exceptions raised by invocation of steps such as XQuery or XSLT should be converted to XD0030, or should such errors pass through so that they can be caught by an XProc p:catch? If the latter, it is odd that XSLT errors XTDE0040 and XTDE0045 should be converted to XC0056.
The WG's position has been that errors do not have to be consistent across implementations; that was viewed as too large a burden to impose. The spec, alas, appears not to say that anywhere. We'll fix that. The error XD0030 is a "standard" escape hatch for errors that don't have a more specific error code. Impls are free to provide more specific/better error codes. We suggest such codes for some errors in some steps.