Raise a ROCrate::ReadException on attempt to read non-RO-Crate zip file

...Previously threw a cryptic `TypeError`

Author: fbacall

lib/ro_crate/reader.rb

@@ -87,6 +87,7 @@ def self.read_zip(source, target_dir: Dir.mktmpdir)
 
       # Traverse the unzipped directory to try and find the crate's root
       root_dir = detect_root_directory(target_dir)
+      raise ROCrate::ReadException, "No metadata found!" unless root_dir
 
       read_directory(root_dir)
     end
@@ -303,7 +304,7 @@ def self.extract_root_entity(entities)
     end
 
     ##
-    # Finds an RO-Crate's root directory (where `ro-crate-metdata.json` is located) within a given directory.
+    # Finds an RO-Crate's root directory (where `ro-crate-metadata.json` is located) within a given directory.
     #
     # @param source [String, ::File, Pathname] The location of the directory.
     # @return [Pathname, nil] The path to the root, or nil if not found.

test/fixtures/just_a_zip.zip

@@ -358,6 +358,11 @@ class ReaderTest < Test::Unit::TestCase
       ROCrate::Reader.read(fixture_file('broken/missing_file'))
     end
     assert_include e.message, 'not found in crate: file1.txt'
+
+    e = check_exception(ROCrate::ReadException) do
+      ROCrate::Reader.read(fixture_file('just_a_zip.zip').path)
+    end
+    assert_include e.message, 'No metadata found'
   end
 
   test 'tolerates arcp identifier on root data entity (and missing hasPart)' do