Replace Errno with dedicated Error types.

Errno is no longer a subclass of Exception in Crystal, instead it is an
Enum and becomes more of an implementation detail.

This set of changes doesn't go as far as it could for detecting the
Errno and making appropriate types, but it does get things compiling
again on the latest version of Crystal.

Author: acook

src/type_checker.cr

@@ -3,13 +3,13 @@
 require "./libmagic/magic"
 
 module Magic
-  class Errno < ::Errno
-    def initialize(msg, checker)
-      msg = <<-ERR
-        #{msg}: libmagic says "#{(err = LibMagic.error(checker)) && String.new(err)}"
-      ERR
-      super msg, Errno.value
-    end
+  class LibMagicError < RuntimeError
+  end
+
+  class NullPointerError < RuntimeError
+  end
+
+  class ReadError < IO::Error
   end
 
   # A TypeChecker checks the Magic database on the system for a given file,
@@ -69,9 +69,9 @@ module Magic
                    limit_settings : Hash(Limit, Int32)? = nil)
       @checker = LibMagic.open @options
       if @checker.null?
-        raise ::Errno.new(
+        raise NullPointerError.from_errno(
           "opening the magic cookie (#{LibMagic.error(@checker)})",
-          ::Errno.value,
+          Errno.value,
         )
       end
       @new_options = @options
@@ -96,7 +96,7 @@ module Magic
       return if chkr === @checker
       LibMagic.close @checker
       @checker = chkr
-      raise error("opening the magic cookie") if @checker.null?
+      raise NullPointerError.new "opening the magic cookie" if @checker.null?
       LibMagic.open @checker
       LibMagic.load @checker, @db_files
       @checker
@@ -135,7 +135,10 @@ module Magic
     end
 
     def error(msg)
-      Errno.new msg, @checker
+      msg = <<-ERR
+        #{msg}: libmagic says "#{(err = LibMagic.error(checker)) && String.new(err)}"
+      ERR
+      LibMagicError.new msg
     end
 
     # Get the filetype "of" the given file, passing `opts` to `libmagic(2)`.
@@ -193,9 +196,9 @@ module Magic
       if (some_bytes.nil? || some_bytes.empty?)
         this.read (some_bytes = Bytes.new(32))
       end
-      raise ::Errno.new "reading bytes, got #{some_bytes.inspect}" if some_bytes.nil? || some_bytes.empty?
+      raise ReadError.new "reading bytes, got #{some_bytes.inspect}" if some_bytes.nil? || some_bytes.empty?
       ptr = LibMagic.buffer(checker, some_bytes, some_bytes.size)
-      String.new ptr || raise error "checking filetype of given byte sequence"
+      String.new ptr || raise NullPointerError.new "checking filetype of given byte sequence"
     end
 
     # same as `#of()`