hush: deal with some easier TODOs

function                                             old     new   delta
is_well_formed_var_name                                -      87     +87
builtin_read                                          49      86     +37
die_if_script                                          -      31     +31
syntax_error_unterminated                              -      28     +28
syntax_error                                          26      51     +25
done_word                                            768     788     +20
syntax_error_at                                        -      12     +12
parse_stream_dquoted                                 320     328      +8
expand_variables                                    2064    2063      -1
run_list                                            1225    1220      -5
add_till_closing_paren                               308     303      -5
add_till_backquote                                   111     106      -5
handle_dollar                                        812     803      -9
parse_stream                                        2378    2356     -22
parse_redirect                                       408     372     -36
maybe_die                                             44       -     -44
is_assignment                                        215     134     -81
------------------------------------------------------------------------------
(add/remove: 4/1 grow/shrink: 4/8 up/down: 248/-208)           Total: 40 bytes

diff --git a/shell/hush_test/hush-arith/arith.right b/shell/hush_test/hush-arith/arith.right
index a35fe89..8cde0ee 100644
--- a/shell/hush_test/hush-arith/arith.right
+++ b/shell/hush_test/hush-arith/arith.right
@@ -55,28 +55,28 @@
 30 30
 20 20
 30 30
-hush: arith: syntax error
+hush: error in arithmetic
 6 6
 6,5,3 6,5,3
 263 263
 255 255
 40 40
-hush: arith: syntax error
-hush: arith: divide by zero
+hush: error in arithmetic
+hush: divide by 0
 hush: can't exec 'let': No such file or directory
-hush: arith: syntax error
+hush: error in arithmetic
 hush: can't exec 'let': No such file or directory
 abc
 def
 ghi
-hush: arith: syntax error
+hush: error in arithmetic
 16 16
-hush: arith: syntax error
-hush: arith: syntax error
-hush: arith: syntax error
+hush: error in arithmetic
+hush: error in arithmetic
+hush: error in arithmetic
 9 9
-hush: arith: syntax error
-hush: arith: syntax error
+hush: error in arithmetic
+hush: error in arithmetic
 9 9
 9 9
 9 9
@@ -97,18 +97,18 @@
 3 3
 4 4
 4 4
-hush: arith: syntax error
-hush: arith: syntax error
-hush: arith: syntax error
-hush: arith: syntax error
-hush: arith: syntax error
+hush: error in arithmetic
+hush: error in arithmetic
+hush: error in arithmetic
+hush: error in arithmetic
+hush: error in arithmetic
 4 4
 7 7
 -7 -7
-hush: arith: syntax error
-hush: arith: syntax error
-hush: arith: syntax error
-hush: arith: syntax error
+hush: error in arithmetic
+hush: error in arithmetic
+hush: error in arithmetic
+hush: error in arithmetic
 6 6
 3 3
 7 7
@@ -119,19 +119,19 @@
 2 2
 -2 -2
 1 1
-hush: arith: syntax error
-hush: arith: syntax error
-hush: arith: syntax error
-hush: arith: syntax error
-hush: arith: syntax error
+hush: error in arithmetic
+hush: error in arithmetic
+hush: error in arithmetic
+hush: error in arithmetic
+hush: error in arithmetic
 5 5
 1 1
 4 4
 0 0
-hush: arith: syntax error
-hush: arith: syntax error
+hush: error in arithmetic
+hush: error in arithmetic
 8 12
-hush: arith: syntax error
+hush: error in arithmetic
 42
 42
 42