UCSC unplaced contigs (chrUn_gl000216) aren't mapped, and one aborts annotation of the whole VCF

Unplaced UCSC contigs (chrUn_gl000216) aren't mapped to Ensembl names, and one such record aborts annotation of the whole VCF.

tests/data/strelka-example.vcf (hg19) has 20 records, two on unplaced contigs:

chrUn_gl000216  76018  .  T             TTTGGG
chrUn_gl000226  1680   .  GACGTATGCACT  G

Both CLI commands fail on it, as does plain load_vcf(...).effects() or .to_dataframe():

$ varcode --vcf tests/data/strelka-example.vcf
...
  File ".../varcode/variant.py", line 422, in gene_ids
    self._check_that_genome_has_contig()
  File ".../varcode/variant.py", line 370, in _check_that_genome_has_contig
ValueError: Invalid contig name 'chrUn_gl000216' for reference 'GRCh37'
$ varcode-genes --vcf tests/data/strelka-example.vcf      # same error from VariantCollection.to_dataframe()

Two problems:

1. No UCSC-to-Ensembl mapping for unplaced or random contigs. _convert_ucsc_contig_name_to_ensembl (varcode/variant.py:591) deliberately skips names containing _. Ensembl 75 does have GL000216.1 (with one gene), and Ensembl 115 has GL000216.2. hg19 chrUn_gl000216 and chr1_gl000191_random correspond to Ensembl GL000216.1 and GL000191.1, and hg38 chrUn_KI270438v1 corresponds to KI270438.1.
2. A contig missing from the genome is fatal for the whole collection. chrUn_gl000226 has no Ensembl GRCh37 contig at all. Alt-haplotype contigs such as chr6_apd_hap1 and decoys such as chrUn_JTFH... are common in real hg38 VCFs. One such record kills VariantCollection.effects() and to_dataframe(), and with them both CLI commands.

Expected: map the UCSC unplaced and random names to their Ensembl accessions. For contigs the genome lacks, warn once (2 variants on contigs not in GRCh37: chrUn_gl000226, ...) and annotate them as Intergenic or skip them, instead of raising.

Start in varcode/variant.py at _convert_ucsc_contig_name_to_ensembl, _check_that_genome_has_contig, and the VariantCollection effects/to_dataframe paths. Reproduce the failure with tests/data/strelka-example.vcf using both CLI commands and load_vcf(...).effects(). Done means unplaced and random UCSC contigs map to Ensembl accessions, while absent contigs produce one warning and are annotated as Intergenic or skipped without aborting the collection.