[t/compiler.t] checked error message on failure

Just knowing that something fails isn't very informative. It turns out
we were sometimes failing for the wrong reasons. This commit is more
discerning about the actual failure.

t/compiler.t

@@ -52,34 +52,36 @@ for @programs-that-compile -> $program {
 }
 
 my @programs-that-don't-compile =   # '
-    '$a',
-    'my',
-    '$a; my $a',
-    'my $a =',
-    '$a = 42',
-    '42 = my $a',
-    '42 := my $a',
-    'say $a',
-    'say $a; my $a',
-    '++42',
-    '{ my $a }; say $a',
-    'else { 42 }',
-    'if 42 say 42',
-    'if $a {}',
-    'if 42 { $a }',
-    'if 5 {} else { $a }',
-    'unless {}',
-    'unless a {}',
-    'my $a = 1; { say $a; my $a = 2 }', # use+declaration is outlawed
+    '$a'            => 'used but not declared',
+    'my'            => 'could not parse',
+    '$a; my $a'     => 'used but not declared',
+    'my $a ='       => 'could not parse',
+    '$a = 42'       => 'used but not declared',
+    '42 = my $a'    => 'could not parse',
+    '42 := my $a'   => 'could not parse',
+    'say $a'        => 'used but not declared',
+    'say $a; my $a' => 'used but not declared',
+    '++42'          => 'could not parse',
+    '{ my $a }; say $a'   => 'used but not declared',
+    'else { 42 }'         => 'could not parse',
+    'if 42 say 42'        => 'could not parse',
+    'if $a {}'            => 'could not parse',
+    'if 42 { $a }'        => 'used but not declared',
+    'if 5 {} else { $a }' => 'used but not declared',
+    'unless {}'           => 'could not parse',
+    'unless a {}'         => 'could not parse',
+    'my $a = 1; { say $a; my $a = 2 }' => 'reference to outer variable',
 ;
 
-for @programs-that-don't-compile -> $program { # '
+for @programs-that-don't-compile -> $pair { # '
+    my ($program, $expected-error) = .key, .value given $pair;
     my $can-compile = False;
     try {
         $c.compile($program);
         $can-compile = True;
     }
-    ok !$can-compile, "will not compile '$program'";
+    ok !$can-compile && defined $!.substr($expected-error),
+        "'{escape $program}' gives error '$expected-error'";
 }
 
 done;