When upgrading to mypy 0.900, most projects will see some errors. Projects
with strict settings could see thousands of errors, since missing stubs
will generate many additional `Any` types. After 200 errors we will only
show errors about unresolved imports or missing stubs so that the likely
root causes won't be hidden in a high volume of errors.

)

When upgrading to mypy 0.900, most projects will see some errors about
missing stubs. Projects with strict settings could see thousands of errors,
since missing stubs will generate many additional Any types.

After 200 errors (only if some of them are about imports) we will now 
only show errors about unresolved imports or missing stubs, so that the 
likely root causes won't be obscured in a high volume of errors.

Fixes #10529.