Lean maxErrors: flag differs by toolchain, weak. form ignored, error lines renamed

On Lean v4.35.0 and v4.36.0-rc2, lean stops after 100 errors; -DmaxErrors=0 removes the cap (700/700 reported), and an in-file set_option does not help. On v4.20.0 and v4.9.0 there is no cap, and -DmaxErrors=N aborts with 'invalid -D parameter, unknown configuration option'. -Dweak.maxErrors=0 is accepted on v4.20.0 but silently ignored on v4.35.0/rc2 (still stops at 100), so pick the flag per toolchain. Also, v4.35 prints ordinary errors as error(lean.unknownIdentifier): ..., so grep -c 'error:' undercounts badly (1 for 100 errors); match ': error' instead.