MutatorMath warnings when generating instances for NotoSansMyanmar

When using fontmake to generate instances, MutatorMath prints warnings. How to fix these?

```sh
$ git clone --recursive https://github.com/googlei18n/noto-source.git
$ ./build setup
$ ./build src/NotoSansMyanmar/NotoSansMyanmar-MM.glyphs -o otf
INFO:fontmake.font_project:Checking Glyphs source for illegal glyph names
INFO:fontmake.font_project:Building master UFOs and designspace from Glyphs source
INFO:glyphsLib:Parsing .glyphs file
INFO:glyphsLib:Casting parsed values
INFO:glyphsLib:Loading to UFOs
INFO:glyphsLib.util:Writing master_ufo/NotoSansMyanmar-Light.ufo
INFO:glyphsLib.util:Writing master_ufo/NotoSansMyanmar-Regular.ufo
INFO:glyphsLib.util:Writing master_ufo/NotoSansMyanmar-SemiBold.ufo
INFO:glyphsLib.util:Writing master_ufo/NotoSansMyanmar-Bold.ufo
INFO:glyphsLib.util:Writing master_ufo/NotoSansMyanmar-CondensedLight.ufo
INFO:glyphsLib.util:Writing master_ufo/NotoSansMyanmar-Condensed.ufo
INFO:glyphsLib.util:Writing master_ufo/NotoSansMyanmar-CondensedSemiBold.ufo
INFO:glyphsLib.util:Writing master_ufo/NotoSansMyanmar-CondensedBold.ufo
INFO:fontmake.font_project:Interpolating master UFOs from designspace
INFO:mutatorMath:Reading master_ufo/NotoSansMyanmar.designspace
INFO:mutatorMath: Generating instance NotoSansMyanmar-Thin.ufo
INFO:mutatorMath:../instance_ufo/NotoSansMyanmar-Thin.ufo:
Errors calculating 41 glyphs:
_ii
_ii.alt
a_m
a_m.sub
a_m.sub2
bbe_mon.sub
bbe_mon.sub2
bha_shn
ca_shn.sub
ca_shn.sub2
ca_shn_dot.sub2
cha
cha.sub2
da_shn.sub
da_shn.sub2
ddha_tlg
dha_tlg
fa_khm_dot
ga_khm
ga_khm.sub
hathi_skn
ja_khm
jha.sub
jha_mon.sub
jha_mon.sub2
nna_epk
nnya.notail
pha_shn
quotedblright
ta.sub2
ta_dot.sub2
tha
tha_dot
tha_dot.sub
tha_dot.sub2
tha_shn
tha_shn.sub
tha_shn_dot
tone3_shn
tone5_rpg
za_shn.sub
INFO:mutatorMath: Generating instance NotoSansMyanmar-ExtraLight.ufo
INFO:mutatorMath:../instance_ufo/NotoSansMyanmar-ExtraLight.ufo:
Errors calculating 41 glyphs:
_ii
_ii.alt
a_m
a_m.sub
a_m.sub2
bbe_mon.sub
bbe_mon.sub2
bha_shn
ca_shn.sub
ca_shn.sub2
ca_shn_dot.sub2
cha
cha.sub2
da_shn.sub
da_shn.sub2
ddha_tlg
dha_tlg
fa_khm_dot
ga_khm
ga_khm.sub
hathi_skn
ja_khm
jha.sub
jha_mon.sub
jha_mon.sub2
nna_epk
nnya.notail
pha_shn
quotedblright
ta.sub2
ta_dot.sub2
tha
tha_dot
tha_dot.sub
tha_dot.sub2
tha_shn
tha_shn.sub
tha_shn_dot
tone3_shn
tone5_rpg
za_shn.sub
```

And so on. Similar warnings are generated for all instances, omitting the full log for brevity.