Add Errno#errno_message getter

src/errno.cr

@@ -204,8 +204,8 @@ class Errno < Exception
   # Returns the numeric value of errno.
   getter errno : Int32
 
-  # Creates a new Errno with the given message. The message will
-  # have concatenated the message denoted by `Errno#value`.
+  # Creates a new `Errno` with the given message. The message will
+  # have concatenated the error message denoted by *errno*.
   #
   # Typical usage:
   #
@@ -217,7 +217,12 @@ class Errno < Exception
   # ```
   def initialize(message, errno = Errno.value)
     @errno = errno
-    super "#{message}: #{String.new(LibC.strerror(errno))}"
+    super "#{message}: #{Errno.error(errno)}"
+  end
+
+  # Returns the error message denoted by *errno*.
+  def self.error(errno = Errno.value)
+    String.new(LibC.strerror(errno))
   end
 
   # Returns the value of libc's errno.
@@ -232,7 +237,7 @@ class Errno < Exception
       LibC.__error.value
     {% elsif flag?(:win32) %}
       ret = LibC._get_errno(out errno)
-      raise Errno.new("get_errno", ret) unless ret == 0
+      raise Errno.new("_get_errno", ret) unless ret == 0
       errno
     {% end %}
   end
@@ -249,7 +254,7 @@ class Errno < Exception
       LibC.__error.value = value
     {% elsif flag?(:win32) %}
       ret = LibC._set_errno(value)
-      raise Errno.new("set_errno", ret) unless ret == 0
+      raise Errno.new("_set_errno", ret) unless ret == 0
       value
     {% end %}
   end