interpreter: improve the error message about install_mode

We wrote:
 ERROR: install_emptydir keyword argument "install_mode" permissions string must
        be exactly 9 characters, got "4" in the form rwxr-xr-x

Let's change this around to be easier to read.

Also, 1-based numbering was used (for components) and 0-based for "bits".
And actually the "bits" are not bits, but octal digits. So say "permissions
character 1", "permissions character 2".

And finally change "must be … if provided" to "can only be". (If it isn't
provided, it "is not", so the sentence is still valid. The user will only
get this error if they provide something, so we don't need to be super precise
and say "if provided". And then we avoid confusing the reader whether
it's "if provided" attaches to the the "False" or to the whole sentence.)

mesonbuild/interpreter/type_checking.py

@@ -44,40 +44,39 @@ def _install_mode_validator(mode: T.List[T.Union[str, bool, int]]) -> T.Optional
 
     This is a rather odd thing, it's a scalar, or an array of 3 values in the form:
     [(str | False), (str | int | False) = False, (str | int | False) = False]
-    Where the second and third arguments are not required, and are considered to
-    default to False.
+    where the second and third components are not required and default to False.
     """
     if not mode:
         return None
     if True in mode:
-        return 'can only be a string or false, not true'
+        return 'components can only be permission strings, numbers, or False'
     if len(mode) > 3:
         return 'may have at most 3 elements'
 
     perms = mode[0]
     if not isinstance(perms, (str, bool)):
-        return 'permissions part must be a string or false'
+        return 'first component must be a permissions string or False'
 
     if isinstance(perms, str):
         if not len(perms) == 9:
-            return (f'permissions string must be exactly 9 characters, got "{len(perms)}" '
-                   'in the form rwxr-xr-x')
+            return ('permissions string must be exactly 9 characters in the form rwxr-xr-x,'
+                    f' got {len(perms)}')
         for i in [0, 3, 6]:
             if perms[i] not in {'-', 'r'}:
-                return f'bit {i} must be "-" or "r", not {perms[i]}'
+                return f'permissions character {i+1} must be "-" or "r", not {perms[i]}'
         for i in [1, 4, 7]:
             if perms[i] not in {'-', 'w'}:
-                return f'bit {i} must be "-" or "w", not {perms[i]}'
+                return f'permissions character {i+1} must be "-" or "w", not {perms[i]}'
         for i in [2, 5]:
             if perms[i] not in {'-', 'x', 's', 'S'}:
-                return f'bit {i} must be "-", "s", "S", or "x", not {perms[i]}'
+                return f'permissions character {i+1} must be "-", "s", "S", or "x", not {perms[i]}'
         if perms[8] not in {'-', 'x', 't', 'T'}:
-            return f'bit 8 must be "-", "t", "T", or "x", not {perms[8]}'
+            return f'permission character 9 must be "-", "t", "T", or "x", not {perms[8]}'
 
         if len(mode) >= 2 and not isinstance(mode[1], (int, str, bool)):
-            return 'second componenent must be a string, number, or False if provided'
+            return 'second componenent can only be a string, number, or False'
         if len(mode) >= 3 and not isinstance(mode[2], (int, str, bool)):
-            return 'third componenent must be a string, number, or False if provided'
+            return 'third componenent can only be a string, number, or False'
 
     return None