The error message
for the missing dependency, including the specifications that had this
dependency.
# File lib/rubygems/resolver/molinillo/lib/molinillo/errors.rb, line 26
def message
sources = required_by.map { |r| "`#{r}`" }.join(' and ')
message = "Unable to find a specification for `#{dependency}`"
message += " depended upon by #{sources}" unless sources.empty?
message
end