Improve the error messages for interpolating undefined or invalid symbols (#1423)

test/asm/interpolation.asm

@@ -1,4 +1,4 @@
-SECTION "Test", ROM0
+SECTION "Test", ROM0[123]
 
 def NAME equs "ITEM"
 def FMT equs "d"
@@ -14,6 +14,15 @@ PRINTLN STRCAT("{NAME}_{d:INDEX}", " is ", {NAME}_{d:INDEX})
 PURGE {NAME}_{d:INDEX}
 ASSERT !DEF({NAME}_{d:INDEX})
 
+; undefined
+PRINTLN "undef {undef}"
+
+; referenced but undefined
+ld hl, label
+PRINTLN "label {label}"
+label::
+PRINTLN "label {label}"
+
 ; not string or number
 MACRO foo
 ENDM

test/asm/interpolation.err

@@ -1,3 +1,7 @@
-error: interpolation.asm(20):
-    Only numerical and string symbols can be interpolated
-error: Assembly aborted (1 error)!
+error: interpolation.asm(18):
+    Interpolated symbol "undef" does not exist
+error: interpolation.asm(22):
+    Interpolated symbol "label" does not exist
+error: interpolation.asm(29):
+    Interpolated symbol "foo" is not a numeric or string symbol
+error: Assembly aborted (3 errors)!

test/asm/interpolation.out

@@ -1,2 +1,5 @@
 ITEM_100 is hundredth
+undef 
+label 
+label $7E
 foo