for `read*()` functions, check if file exists and output and error if it doesn't

[taylorreiter]

Otherwise, when a non-existent file is supplied:

> Sys.glob("~/github/2022-strains/SRR/SRR*lineage*csv")
character(0)

You get this entirely unhelpful error message:

Error in tidyr::separate(., .data$lineage, into = c("domain", "phylum", : 
object 'taxonomy_annotate_df' not found

[taylorreiter]

but...do this cleverly so that if read from URL still works, this check doesn't break that functionality

[taylorreiter]

Addressed in #61, including keeping read url functionality