Possible bug in GlyphsLib when defining the default master into designspace file

## Observed behaviour

When trying to change the default value for the Weight axis from Thin to Light, it was not possible to generate the Variable Font due to a possible bug in GlyphsLib with the Default value or master.

Process followed (and reviewed with @m4rc1e):

1. Source file without Axis Mapping
fontmake -g BigShoulders.glyphs -o variable
R: `Error. Source or instance ‘Big Shoulders Thin’ has out-of-range location for axis ‘Optical size’`

2. Adding Axis Mapping Custom Parameter + Variable Font Origin
fontmake -g BigShoulders.glyphs -o variable
R: `Error: there must be a mapping for the axis default value 333.59375`

3. Changing manually the default value to 300
fontmaker -m BigShoulders.designspace -o variable
R: `Error: In 'vf.designspace': Generating fonts from Designspace failed: Can't find base (neutral) master in DesignSpace document`

The file has equal names for two masters (for 10pt and 72pt), but when assigning different names, to rule it out as a possible source of trouble, we got:
`Error: Generating fonts from Designspace failed: More than one base master found in Designspace. `

## Expected behaviour

To obtain a working designspace file with valid default master that Fontmake could identify.
To have a more informative error message that could help to solve it

## Resources and exact process needed to replicate

[Big_Shoulders.glyphs.zip](https://github.com/googlefonts/glyphsLib/files/5358165/Big_Shoulders.glyphs.zip)

Start by reproducing the three fontmake commands with the attached Big_Shoulders.glyphs.zip and compare the generated designspace files. Inspect GlyphsLib's handling of the default value and master while converting the Glyphs source; done means fontmake can identify one valid default master and the error explains invalid mappings clearly.

Written by the indexing model from the issue text.