Remove location info from `Exception#inspect`

Because location info (file name and line number) is kept in the backtrace,
it should not be kept in the result of `inspect` (and the exception object
itself), I think.

### Example

  ```ruby
  # example.rb
  begin
    raise "err"
  rescue => e
    p e
  end
  ```

#### Before this patch:

  ```
  $ bin/mruby example.rb
  example.rb:2: err (RuntimeError)
  ```

#### After this patch:

  ```
  $ bin/mruby example.rb
  err (RuntimeError)
  ```

1 files changed, 3 insertions, 1 deletions

diff --git a/test/t/exception.rb b/test/t/exception.rb
index bdf277c1e..a6f69eef6 100644
--- a/test/t/exception.rb
+++ b/test/t/exception.rb
@@ -352,8 +352,10 @@ assert('Exception 19') do
   assert_equal [true, true], Class4Exception19.new.a
 end
 
-assert('Exception#inspect without message') do
+assert('Exception#inspect') do
   assert_equal "Exception", Exception.new.inspect
+  assert_equal "Exception", Exception.new("").inspect
+  assert_equal "error! (Exception)", Exception.new("error!").inspect
 end
 
 assert('Exception#backtrace') do