Improves error reporting

For function headers with wrong typing
this catched the error and recovers better, preventing
further spurious (fake) errors.

Author: boriel

src/libzxbc/zxbparser.py

@@ -2911,6 +2911,13 @@ def p_function_header(p):
     p[0] = p[1]
 
 
+def p_function_header_error(p):
+    """ function_header : function_def error CO
+                        | function_def error NEWLINE
+    """
+    p[0] = None
+
+
 def p_function_header_pre(p):
     """ function_header_pre : function_def param_decl typedef
     """

tests/functional/test_errmsg.txt

@@ -176,6 +176,9 @@ prepro76.bi:2: error: this is an intended error
 line_asm.bi:26: warning: this should be line 26
 >>> process_file('line_err.bas')
 line_err.bas:5: error: Variable 'q' already declared at line_err.bas:1
+>>> process_file('let_expr_type_crash.bas')
+let_expr_type_crash.bas:3: error: Syntax Error. Unexpected token 's' <ID>
+let_expr_type_crash.bas:8: error: Function 'editStringFN' takes 0 parameters, not 3
 
 # Test warning silencing
 >>> process_file('mcleod3.bas', ['-S', '-q', '-O --expect-warnings=2'])
@@ -184,3 +187,4 @@ mcleod3.bas:3: error: 'GenerateSpaces' is neither an array nor a function.
 >>> process_file('doloop2.bas', ['-S', '-q', '-O -W110'])
 doloop2.bas:4: warning: [W100] Using default implicit type 'ubyte' for 'a'
 doloop2.bas:4: warning: [W150] Variable 'a' is never used
+