[Ada] Improve error message on mult/div between fixed-point and integer

[Ada] Improve error message on mult/div between fixed-point and integer

Multiplication and division of a fixed-point type by an integer type is
only defined by default for type Integer. Clarify the error message to
explain that a conversion is needed in other cases.

Also change an error message to start with lowercase as it should be.

2019-07-01  Yannick Moy  <moy@adacore.com>

gcc/ada/

	* sem_ch4.adb (Operator_Check): Refine error message.

From-SVN: r272866