Added parse errors for %USE calls

So %USE <STRING> is the expected call pattern, so there's an error if
there isn't a string after %USE.

The other two errors are file I/O errors i.e. nonexistent files or
errors in parsing the other file.  We don't report specifics about the
other file, that should be up to the user to check themselves.

asm/parser.c

@@ -39,6 +39,12 @@ const char *perr_as_cstr(perr_t perr)
     return "EXPECTED_LABEL";
   case PERR_EXPECTED_OPERAND:
     return "EXPECTED_OPERAND";
+  case PERR_PREPROCESSOR_EXPECTED_STRING:
+    return "PREPROCESSOR_EXPECTED_STRING";
+  case PERR_PREPROCESSOR_FILE_NONEXISTENT:
+    return "PREPROCESSOR_FILE_NONEXISTENT";
+  case PERR_PREPROCESSOR_FILE_PARSE_ERROR:
+    return "PREPROCESSOR_FILE_PARSE_ERROR";
   case PERR_PREPROCESSOR_EXPECTED_END:
     return "PREPROCESSOR_EXPECTED_END";
   case PERR_PREPROCESSOR_EXPECTED_NAME:

asm/parser.h

@@ -28,7 +28,10 @@ typedef enum
   PERR_EXPECTED_LABEL,
   PERR_EXPECTED_OPERAND,
   PERR_PREPROCESSOR_EXPECTED_NAME,
+  PERR_PREPROCESSOR_EXPECTED_STRING,
   PERR_PREPROCESSOR_EXPECTED_END,
+  PERR_PREPROCESSOR_FILE_NONEXISTENT,
+  PERR_PREPROCESSOR_FILE_PARSE_ERROR,
   PERR_PREPROCESSOR_UNKNOWN_NAME,
   PERR_INVALID_RELATIVE_ADDRESS,
   PERR_UNKNOWN_OPERATOR,