USDM rules with more than one merged dataset fail since #1445

[ASL-rmarshall]

The changes implemented in PR #1445 have broken all USDM rules that merge more than one dataset. The issue is that the error handling references domain, which is not relevant for USDM datasets. Merging of the second and subsequent dataset is skipped due to

cdisc-rules-engine/cdisc_rules_engine/utilities/dataset_preprocessor.py

Lines 118 to 119 in e785d2d

	if file_info.domain in merged_domains:
	continue

because the file_info.domain added to merged_datasets here

cdisc-rules-engine/cdisc_rules_engine/utilities/dataset_preprocessor.py

Line 165 in e785d2d

merged_domains.add(file_info.domain)

is None for all USDM datasets.