interpreter: improve the error message about install_mode
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/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,pexip/meson,pexip/meson,pexip/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,pexip/meson,pexip/meson,mesonbuild/meson