Shorten error messages

Author: AA-Turner

flit_core/flit_core/config.py

@@ -854,7 +854,7 @@ def normalise_compound_license_expr(s: str) -> str:
                 raise ConfigError(invalid_msg.format(s=s, reason=reason))
             elif part == '(':
                 if parts and parts[-1] not in {' AND ', ' OR ', '('}:
-                    reason = "'(' must follow a license expression operator or another '('"
+                    reason = f"'(' must follow either AND, OR, or '('"
                     raise ConfigError(invalid_msg.format(s=s, reason=reason))
                 stack += 1
                 parts.append(part)
@@ -869,7 +869,7 @@ def normalise_compound_license_expr(s: str) -> str:
                 parts.append(part)
             else:
                 if parts and parts[-1] not in {' AND ', ' OR ', '('}:
-                    reason = "a license ID must follow a license expression operator or '('"
+                    reason = f"a license ID must follow either AND, OR, or '('"
                     raise ConfigError(invalid_msg.format(s=s, reason=reason))
                 simple_expr = normalise_simple_license_expr(part)
                 parts.append(simple_expr)